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v  sge is a blue star with a mean brightness around 11 mag which has been shown to vary between 9.614.7 mag since its discovery in 1902 . it shows wide eclipses at a period of 051419 , a small secondary eclipse , and complex emission line behaviour ( herbig  1965 ) . extinction estimates vary between @xmath3 ( herbig  1965 ) and @xmath4 ( verbunt 1987 ) implying a distance of 0.72.7 kpc . supersoft x - ray binaries ( ssb ; see greiner 1996 and references therein ; van teeseling 1998 ) were established as a new class of accreting binaries during the early 90ies with rosat ( trmper  1991 ; greiner  1991 ) and are thought to contain white dwarfs accreting mass at rates sufficiently high to allow stable nuclear surface burning of the accreted matter ( van den heuvel  1992 ) . ssb have luminosities of @xmath5  10@xmath610@xmath7 ergs s@xmath2 , but their characteristic temperatures of 2040 ev imply strong attenuation by the interstellar medium . thus , most of the known ssb are located in external galaxies ( e.g. greiner 1996 ) making detailed optical observations difficult . it is therefore of great interest to identify galactic ssb . it has recently been suggested ( steiner & diaz 1998 ; patterson  1998 ) that v  sge has spectroscopic and photometric properties which are very similar to those of ssb . this suggestion is based on characteristics which are typical for ssb , but are rare or even absent among canonical cataclysmic variables : ( 1 ) the presence of both o  and n  emission lines , ( 2 ) a he@xmath8/h@xmath9 emission line ratio @xmath10 , ( 3 ) rather high absolute magnitudes and very blue colours , and ( 4 ) orbital lightcurves which are characterized by a wide and deep eclipse . the suggestion of the similarity of v  sge to ssb is almost entirely based on optical and ultraviolet data . in this paper , we investigate the archival rosat data of v  sge and discuss them in the context of the long - term optical behaviour of v  sge . hoard  ( 1996 ) reported the detection of v  sge as a soft x - ray source in the nov . 1992 rosat observation , but did not perform a spectral fit . verbunt et al . ( 1997 ) already reported the non - detection of v  sge during the rosat all - sky survey . clrccccccr date &  obs - id@xmath11 & t@xmath12   & offaxis & cr@xmath13 & hr1@xmath14 & hr2@xmath14 & x - ray & optical & d@xmath15 + & & ( sec )   & angle & ( cts / s ) & & & state & state & + oct . 1931 , 1990 & &  50 & 055 & @xmath160.054 &  &  &  & bright &    + nov . 23/24 , 1991 & 400155p & @xmath1710235 & 039 & 0.0011@xmath180.0004 & 1.0@xmath180.7 & 0.2@xmath180.4 & hard & bright & @xmath1916 + nov . 1012 , 1992 & 300182p & @xmath1727745 & 309 & 0.0091@xmath180.0010 & 0.64@xmath180.15 & 0.13@xmath180.39 & soft & intermediate & 28 + apr . 1924 , 1994 & 300311h & @xmath1724610 & 018 & @xmath160.00044 &  &  &  & bright &    + may 1213 , 1994 & 300311h & 4700 & 018 & 0.0199@xmath180.0021 &  &  & very soft & faint & 1 + oct . 18/19 , 1994 & 300311h-1 & @xmath1718440 & 017 & 0.0011@xmath180.0003 &  &  & hard & bright & 5 + may 12/13 , 1997 & 300582h & @xmath1715700 & 017 & 0.0025@xmath180.0004 &  &  & soft & intermediate & 1 +    @xmath11 the letter after the observation i d number gives the rosat detector : p = pspc , h = hri . + @xmath13 count rates in the corresponding detector in the 0.12.4 kev range ( pspc : channels 11240 ) . upper limits are 3@xmath20 confidence level . note the different pspc to hri count rate conversion factors of 2.7:1 and 7.8:1 for hard and soft spectrum sources . + @xmath14 hardness ratios with hr1 = @xmath21 and hr2 = @xmath22 , where @xmath23 kev ) , @xmath24 kev ) , @xmath25 kev ) , and @xmath26 kev ) are the counts in the given energy range . + @xmath15 distance between best - fit x - ray and optical position . for the optical position @xmath27(2000.0)=2020147 , @xmath28(2000.0)=+210610  has been used as determined from the second generation dss . this position differs from the simbad position by @xmath29=12  and @xmath30=4 . [ log ]    v  sge has been the target of three dedicated pointed pspc and hri observations ( one of these splits into 3 separate observation intervals ) , and in addition is in the field of view of another pspc observation ( table  [ log ] ) . the results of these observations are quite diverse : v  sge has not been detected during the rosat all - sky survey in 1990 and a long rosat hri pointing in april 1994 , but has been detected during all other observations , even in a much shorter hri observation . thus , v  sge shows strong x - ray variability with an amplitude of a factor of 140 . in addition , the x - ray spectral characteristics during two rosat pspc pointings obtained 1 yr apart show a remarkable difference : at one occasion v  sge has a ` supersoft ' x - ray spectrum , at another occasion the spectrum is very hard . the diversity of x - ray measurements looks more ordered when it is compared with the optical brightness of v  sge . this binary system is included in the roboscope program of automatic long - term monitoring the results of which led to the classification of three distinct optical states : bright state ( v@xmath1611 mag ) , intermediate state ( v@xmath311112 mag ) and faint state ( v@xmath3212 mag ) ( robertson  1997 ) . we have combined the optical lightcurve obtained by these observations with data collected in the vsolj database ( www.kusastro.kyoto-u.ac.jp/vsnet/ ) and plotted these in fig . [ lc ] together with the times of the rosat observations . this suggests that during optical bright state v  sge is a hard , but rather faint x - ray source , while during optical faint state v  sge is a more luminous and very soft x - ray source . during the intermediate optical state also the x - ray spectrum is intermediate with respect to the very soft and hard spectrum . to obtain an idea about the x - ray spectral parameters during the soft x - ray state , we fit the nov . 1992 pspc spectrum with a solar - abundance lte @xmath33 white dwarf atmosphere model ( van teeseling  1994 ) . the @xmath34 contours are shown in fig . [ logg9 ] . the @xmath35 contour suggests a temperature @xmath36k and a bolometric luminosity @xmath37ergs@xmath2 ( with @xmath38kpc ) , but lower temperatures and higher luminosities are still acceptable within the 90% confidence contour . if we require that the soft x - ray absorbing column is at least @xmath39@xmath40 as derived from the @xmath41absorption dip ( @xmath42 ; verbunt 1987 ) , we find @xmath43k and @xmath44ergs@xmath2 . with @xmath45@xmath40 , only for @xmath46200000k a luminosity of @xmath47ergs@xmath2 is reached . it is possible , however , that because of the very high orbital inclination the soft x - ray absorption is much larger than the ultraviolet absorption . a similar discrepancy is known for cal87 ( cf . hutchings  1995 ; parmar  1997 ) . if we relax the absorption constraint and assume that v  sge is a ssb with @xmath47ergs@xmath2 , we find @xmath48k and a radius @xmath49 cm consistent with a white dwarf . however , because of a very high orbital inclination , the white dwarf may be completely obscured from view by the accretion disk rim , in which case the observable luminosity ( from x - rays scattered into the line of sight ) may be much less than @xmath50ergs@xmath2 . we note that there is no significant modulation of the soft x - rays on the orbital period . a factor of 45 increase in hri count rate occurred within less than three weeks in april / may 1994 during which the optical brightness decreased and v  sge eventually became a very soft x - ray source . though the rosat observations during this optical state transition have been performed with the hri , the grossly different spectral shapes are easy to recognize ( fig . [ logg9 ] , bottom ) . the anti - correlation of soft x - ray emission with optical brightness is reminiscent of the behaviour of the ssb 513(reinsch  1996 ; southwell  1996 ) . 513  turns on as a supersoft x - ray source only during @xmath51mag optical dips , which occur every 100200 days and last about @xmath52 days . this behaviour has been explained by assuming that the shell - burning white dwarf in 513  has normally expanded to a few @xmath53 cm and radiates its luminosity in the extreme - ultraviolet . during the optical faint states , the white dwarf contracts with almost constant bolometric luminosity to @xmath54 cm , and radiates in the soft x - ray band . the model that has been suggested for 513cannot explain the observational data of v  sge . first , the optical brightness changes of v  sge are very rapid : both the faint-/bright - state transitions as well as the succession of different faint states may occur on timescales of @xmath55day ( compared to the smooth decline of several days in 513 ) . such very rapid changes are only possible if the white dwarf envelope expands and contracts on the kelvin - helmholtz timescale and the mass of the expanding envelope is rather small ( @xmath56 ) . such a small envelope mass is difficult to accept for a white dwarf with stable shell burning ( e.g. prialnik & kovetz 1995 ) . second , the expected optical eclipse would become deeper when the system becomes brighter , opposite to what has been observed ( patterson  1998 ) .    before we speculate on a possible explanation for the observed x - ray properties of v  sge , we note the similarity of the v  sge behaviour to that of vy  scl stars ( as has been noted with respect to the optical behaviour also by robertson  1997 ) . in a recent survey of the available rosat data of vy  scl stars ( greiner 1998 ) a relatively hard x - ray spectrum was found for vy scl stars during optical bright state ( see also van teeseling  1996 ) . moreover , observations of the vy  scl star v751  cyg during its 1997 optical faint state have revealed luminous and very soft x - ray emission ( greiner  1998 ) , similar to that of v  sge in its faint state . inspection of the change in eclipse depth from faint to bright state ( e.g. fig .  5 in patterson  1998 ) shows that it is possible to reproduce this change by an increase of uneclipsed flux , while the eclipsed light ( presumably from the irradiated accretion disk ) remains almost constant . if the flux from the irradiated disk ( and therefore also from the irradiated secondary ) remains unchanged , this would suggest that a brightening of v  sge is caused by an increasing amount of extended luminous ( outflowing ? ) matter . this would also be consistent with the emission lines , which only show partial eclipse effects , and which increase in strength when v  sge brightens ( e.g. herbig  1965 ) , indicating either an increasing amount of line emitting matter or an increasing amount of ionizing flux . ( mauche  ( 1997 ) note that the heii@xmath57/civ@xmath58 and nv@xmath59/civ@xmath58 line ratios decrease when v  sge brightens . )    additional extended gas could increase the amount of soft x - ray absorption and make the soft x - ray component fainter and harder and make it even completely undetectable . because it is impossible to produce the hardness ratios _ and _ count rate observed in nov . 1991 by simply adding more absorption to an absorbed hot - white - dwarf spectrum , we conclude that in nov . 1991 an additional harder x - ray component was present . the hardness ratios and count rate of the nov . 1991 pspc observation can be explained perfectly with a thermal bremsstrahlung spectrum , absorbed with @xmath60@xmath40 , a temperature of a few kev , and a 0.12.4kev luminosity of @xmath1ergs@xmath2 ( at 1kpc ) . the same bremsstrahlung component may also have been present during the soft x - ray state in nov . 1992 without significantly affecting the confidence intervals in fig . [ logg9 ] of the spectral parameters of the soft component . we note that such a bremsstrahlung component is not inconsistent with the x - ray flux of an evolved secondary in a 12hr binary ( cf . dempsey 1993 ) .    a simple wind model for the recently observed radio flux density of v  sge implies a mass - loss rate of the order of 10@xmath61/yr ( lockley  1997 ) . with their ( assumed ) terminal velocity of 1500 km / s this wind zone is completely opaque for x - rays up to 0.7kev , even if the wind is assumed to be circumbinary instead of arising from one component . since the radio measurement has been obtained during optical high state , it supports the above described scenario . we note that the colliding wind scenario as discussed in lockley  ( 1997 ) would predict a positive correlation between optical and x - ray emission , contrary to our finding . vitello & shlosman ( 1993 ) have modeled the uv line shapes of v  sge assuming a biconical accretion disk wind , and need high mass - loss rates to explain the observations , while they did not consider the possibility of a luminous shell - burning white dwarf . we also note that irradiation - induced winds with a rate of 10@xmath6210@xmath61/yr are expected in ssb ( van teeseling & king 1998 ) . we conclude that it is possible to explain the optical and x - ray behaviour of v  sge by assuming a variable amount of extended uneclipsed matter which contributes significantly in the optical ( by reprocessing of soft x - rays ? ) and may completely absorb the ` supersoft ' x - ray component during the optical bright / hard x - ray state . the x - ray properties of v sge , in any case , support the presence of a hot luminous white dwarf , possibly with hydrogen shell - burning , and do not hinder the addition of v sge to the class of supersoft x - ray binaries . more detailed modelling of the changing optical light curves and ultraviolet and optical spectra are necessary to test this scenario . we thank kent honeycutt for providing his roboscope data as published in robertson  ( 1997 ) in electronic format . jg and avt are supported by the german bundesministerium fr bildung , wissenschaft , forschung und technologie ( bmbf / dlr ) under contract no . fkz 50 qq 9602 3 and 50or96098 , respectively . the project is supported by bmbf / dlr and the max - planck - society . this research has made use of the simbad database , operated at cds , strasbourg , france and the digitized sky survey ( dss ) produced at the space telescope science institute under us government grant nag w-2166 .
we investigate the rosat x - ray properties of v  sge , which has been proposed to be related to supersoft x - ray binaries . during optical bright states , v  sge is a faint hard x - ray source , while during optical faint states ( @xmath0 mag ) , v  sge is a ` supersoft ' x - ray source . 
 spectral fitting confirms that v  sge s x - ray properties during its soft x - ray state may be similar to those of supersoft x - ray binaries , although a much lower luminosity can not be excluded . 
 it is possible to explain the different optical / x - ray states by a variable amount of extended uneclipsed matter , which during the optical bright states contributes significantly to the optical flux and completely absorbes the soft x - ray component . 
 an additional , perhaps permanent , hard x - ray component , such as a bremsstrahlung component with a 0.12.4kev luminosity of @xmath1ergs@xmath2 , must be present to explain the x - ray properties during the optical bright / hard x - ray state .