The Post–Common Envelope and Pre–Cataclysmic Binary PG 1224+309

We have made extensive spectroscopic and photometric observations of PG1224+309, a close binary containing a DA white dwarf primary and an M4+secondary. The H alpha line is in emission due to irradiation of the M-star bythe hot white dwarf and is seen to vary around the orbit. From the radialvelocities of the H alpha line we derive a period of P = 0.258689 +/- 0.4days and a semi-amplitude of K_Halpha = 160 +/- 8 km/s. We estimate acorrection Delta_K = 21 +/- 2 km/s, where K_M = K_Halpha + Delta_K. Radialvelocity variations of the white dwarf reveal a semi-amplitude of K_WD = 112+/- 14 km/s. The blue spectrum of the white dwarf is well fit by a syntheticspectrum having T_eff = 29,300 K and log(g) = 7.38. The white dwarf contributes97% of the light at 4500 Angstroms and virtually all of the light blueward of3800 Angstroms. No eclipses are observed. The mass inferred for the white dwarfdepends on the assumed mass of the thin residual hydrogen envelope: 0.40 < M_WD< 0.45 solar masses for hydrogen envelope masses of 0 < M_H < 4.0E-4 solarmasses. We argue that the mass of the white dwarf is closer to 0.45 solarmasses, hence it appears that the white dwarf has a relatively large residualhydrogen envelope. The mass of the M-star is then M_M = 0.28 +/- 0.05 solarmasses, and the inclination is i = 77 +/- 7 degrees. We discuss briefly how PG1224+309 may be used to constrain theories of close binary star evolution, andthe past and future histories of PG 1224+309 itself. The star is both a``post-common envelope'' star and a ``pre-cataclysmic binary'' star. Masstransfer by Roche-lobe overflow should commence in about 10 Gyr.Comment: 17 pages, 8 figures, AAS LaTeX, to appear in AJ, March 199