4 Discussion

The presence of weak emission lines in the infrared (and optical) spectrum of MR Vel, the detection of highly ionized elements like He II and O VI, and the fact that the secondary star remains hidden in the IR spectrum, are consistent with a very luminous accretion disk. Are the accretion disks in longer orbital period systems more luminous? This is possible, since the thermal time scale for the secondary, regulating the mass transfer, decreases with the square of the donor mass. For instance, in QR And, whose orbital period is only 15.85 h, the He II  $\lambda \lambda$ 16926 and 21891 equivalent widths (Quaintrell & Fender 1998) are a factor 3 larger than in MR Vel, consistent with a fainter disk.

Interestingly, a P-Cygni profile has been observed in MR Vel at lines with very different excitation potential. The feature has been observed in optical H I lines (Matsumoto & Mennickent 2000; Schmidtke et al. 2000) and in the infrared (this paper). P-Cygni absorptions are also observed in X-ray lines like Fe XVII and O VIII (Bearda et al. 2002) with approaching velocities reaching a few thousands km s^-1. It is not clear, at present, if the visibility of the P-Cygni profiles in the spectrum depends on the orbital phase. One may speculate that the less dense upper parts of the disk can be pushed away by the radiation pressure arising from the inner disk, producing an extended wind region where the P-Cygni absorptions arise.

It is interesting to note that the slope of the infrared continuum reported in Sect. 3 differs from the slope of an infinitely large steady state accretion disk ( $F \propto \lambda^{-7/3}$Lynden-Bell 1969). This is likely related to a non-neglectable contribution of the secondary star to the overall flux in the infrared.

Our distance estimate is consistent with the 4 kpc value given by Hartmann et al. (1999) and only marginally consistent with the range derived by Motch et al. (1994), viz.  1-2 kpc. The distance derived using the accretion disk model fitting suggests an absolute magnitude $M_{\rm V}$ between -2 and 0. Due to the nature of the assumptions considered in our modeling, this value should be taken with extreme caution. This is especially important, since in our model the secondary should contribute about 40% to the total light of the system in the infrared, however their spectral features are not visible in our spectra.