  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 and a massive white dwarf. 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. 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. The projected rotational velocity is determined for the first time
from 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
vrotsini = 92.3 +/- 1.5 km/s is determined. 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.
