M = 0.45 Jupiter-Masses / sin i
where i is the inclination of the orbit (0 for face-on).
The statistical probability that the orbital plane is inclined sufficiently face-on that M > 50 MJUP is: Probability = 0.0001 .
Since Mayor's group and we have observed only 260 stars, it is unlikely that such an extreme face-on orientation would occur.
More importantly, the weak chromospheric and coronal emission of 51 Peg rule out a close stellar companion. The weak corona and chromosphere imply a spin period of, Prot ~ 35 d. This slow spin period for the star (compared to the planet's orbital period of 4.2 day) shows that the planet orbits the star faster than 51 Peg spins on axis. This slow spin rate for the star rules out significant tidal coupling between the planet and the star.
The suggestion that the differential rotation might not occur for tidally-coupled stars (thereby artificially supressing the magnetic-driven chromosphere) is empirically not born out. Many close STELLAR binaries are known having orbital periods of 2 - 6 days, the same orbital periods as 51 Peg. These Solar-type star binaries have X-ray luminosities (ROSAT) of typically:
Log(Lx) = 29. for G-dwarf binaries
log(Lx) = 27.0 for 51 Peg
The coronal X-ray flux from 51 Peg is too low, inconsistent (at the 5-sigma level) with a close stellar companion. Again, the companion obviously can't be a star. For a thorough discussion of this, see: Pravdo et al. 1996, New Astronomy, vol 1., p 171.
Further, its age of ~8Gyr implies that tidal coupling would have easily occurred in 51 Peg. But it hasn't, thus ruling out a close stellar companion.