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tokens: deficit due to a gap (or hole) opened by the secondary black hole migrating within the circumbinary disk. Given the morphological evidence that Mrk 231 is indeed a recent merger (e.g., Hamilton Keel 1987 ; Hutchings Neff 1987 ; Surace et al. 1998 ; Veilleux et al. 2002 , 2006 ), and the dearth so far of black hole binaries in the cores of quasars (e.g., Eracleous et al. 2012 ; Popović 2012 ; Shen et al. 2013 ; Liu et al. 2014 ; Runnoe et al. 2015 ), contrary to theoretical expectations (Begelman et al. 1980 ; Yu 2002 ; Merritt Milosavljević 2005 ), the binary black hole explanation for the peculiar SED of Mrk 231 is potentially a very exciting new explanation of the Cycle 19 data, so it deserves to be examined in detail taking into account the new constraints from the Cycle 21 data. As pointed out by Yan et al. ( 2015 ), numerical simulations of black hole binaries (e.g., Miller Krolik 2013 ; Farris et al. 2014a , 2014b ; Roedig et al. 2014 ) show that the accretion onto the secondary black hole is expected to vary on a timescale of the order of the black hole binary orbital period, t orb ∼ 1.2 years, or less. However, as described here and in Paper I , the UV continuum emission in Mrk 231 has been surprisingly constant over the last several years. There is no evidence for any flux variations in the FUV continuum emission between 2011 (Paper I ) and 2014 (this paper), or ∼3 × t orb , and in the NUV continuum emission between 1992 (Smith et al. 1995 ) and 2014 (this paper), or ∼18 × t orb . This is not to say that the UV continuum of Mrk 231 does not vary at all. In fact, there is some evidence that the continuum level near 1300 Å in the 1978 archival International Ultraviolet Explorer ( IUE ) spectra of Mrk 231 (published in Hutchings Neff 1987 ) was ∼1.5× higher than the continuum level measured in our recent COS FUV data. But, the binary black hole model has difficulties explaining the stability of the UV emission in recent years. It would imply a (unrealistically) steady accretion stream of gas from the inner edge of the circumnuclear binary to the mini-disk of the secondary black hole over timescales of ∼3–18 t orb . A potentially fatal hurdle of the binary black hole model is the stability of the Ly α profile over a period of ∼3 year (∼3 × t orb ). In this model, the mini-disk of the secondary black hole is expected to dominate the line emission in the FUV, and the large blueshift of Ly α , C iii ], and C iv would therefore be due to the expected large line of sight velocity of the secondary black hole as it orbits the primary black hole. However, if this were the case, one would expect the centroid of the Ly α profile to shift in velocity from –3000 to at least +3000 km s −1 over half an orbital period. This is firmly ruled out by the new Cycle 21 data. Another difficulty with the binary black hole model is the fact that the absorption lines are only observed in the NUV and optical, but not in the FUV. This means that the BALR must be