tokens: of the maser emission, the spots could be bright at different times, and thus we consider the fit with nonzero positional offset to be more robust. Second, we performed an astrometric fit to the mean position of the shock front and derive a trigonometric parallax of π = 6.6 ± 0.6 mas, corresponding to a distance of pc. We show the parallax sinusoid from this fit in Fig. 6 , and give the resulting parameters in Table 5 . The values of the parallax derived from the fits to the three main spots are in good agreement with that obtained from the fit to the R.A. motion of the shock front. Table 5 Parameters obtained from the astrometric fits. The systematic errors that were added quadratically are also presented. The correlation coefficient matrices for all fits are shown in Appendix A . From these matrices we noticed a high (anti-) correlation between the parallax and the other fitted parameters in all the fits, as is expected given the short timescale covered by the observations. To check the reliability of our results, in view of these high correlations, we performed a series of Monte Carlo simulations which are presented in Appendix B . The result from these simulations is that, given the statistical and systematic errors in our measurements, the true astrometric parameters can be recovered from our fits even when the observations cover a period of only 3–7 months. The worst case is the individual fit to spot 1; this spot was only detected in four epochs and, consequently, the uncertainties on the fitted parameters are significantly larger. 4 Discussion 4.1 Distance to I16293 As we mentioned in the introduction, Imai et al. (2007) obtained a distance of 178 pc to I16293 by measuring the trigonometric parallax of water masers associated with this YSO. Imai et al. (2007) mentioned that the most severe factor causing their modest astrometric accuracy was the temporal variation in the brightness structure of the maser spots they detected. If water masers have short lifetimes of a few months, as is the case with water masers in I16293, it is possible to misidentify maser spots even from one epoch to the next. Taking into account this possibility, these authors performed several fits by using different combinations of observed positions. By excluding the oddest positions, they obtained an upper limit to the trigonometric parallax of 7.1 mas (corresponding to a lower limit in the distance of d = 141 pc). In the following, we discuss the differences between our analysis and that of Imai et al. (2007) . Fig. 6 Parallax fit to the mean positions of the shock front. Top: parallax sinusoid from the best fit to these data (blue line). The red squares are the mean positions of the shock front. In both cases, we have subtracted the proper motion and the RA position at JD 2453731.31 (the mean epoch from our observations) from the best fit. Bottom : residuals in right ascension. The first difference comes from their criterion for the selection of maser
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