  The nature of the progenitors of type Ia supernovae is still under
controversial debate. KPD 1930+2752 is one of the best SN Ia progenitor
candidates known today. The object is a double degenerate system consisting of
a subluminous B star (sdB) and a massive white dwarf (WD). Maxted et al. (2000)
conclude that the system mass exceeds the Chandrasekhar mass. This conclusion,
however, rests on the assumption that the sdB mass is 0.5 Mo. However, recent
binary population synthesis calculations suggest that the mass of an sdB star
may range from 0.3 Mo to more than 0.7 Mo. It is therefore important to measure
the mass of the sdB star simultaneously with that of the white dwarf. Since the
rotation of the sdB star is tidally locked to the orbit the inclination of the
system can be constrained if the sdB radius and the projected rotational
velocity can be measured with high precision. An analysis of the ellipsoidal
variations in the light curve allows to tighten the constraints derived from
spectroscopy. We derive the mass-radius relation for the sdB star from a
quantitative spectral analysis of 150 low-resolution spectra. The projected
rotational velocity is determined for the first time from 200 high-resolution
spectra. In addition a reanalysis of the published light curve is performed.
The atmospheric and orbital parameters are measured with unprecedented
accuracy. In particular the projected rotational velocity is determined. A
neutron star companion can be ruled out and the mass of the sdB is limited
between 0.45 Mo and 0.52 Mo. The total mass of the system ranges from 1.36 Mo
to 1.48 Mo and hence is likely to exceed the Chandrasekhar mass. So KPD
1930+2752 qualifies as an excellent double degenerate supernova Ia progenitor
candidate.
