The post-common envelope and pre-cataclysmic binary PG 1224+309

We have made extensive spectroscopic and photometric observations of PG 122 4 +/- 309, a close binary containing a DA white dwarf primary and an M4+ se condary. The H alpha line is in emission due to irradiation of the M star b y the hot white dwarf and is seen to vary around the orbit. From the radial velocities of the H alpha line we derive a period of P = 0.258689 +/- 0.00 0004 days and a semiamplitude of K-H alpha = 160 +/- 8 km s(-1). We estimat e a correction Delta K = 21 +/- 2 km s(-1), where K-M = K-H alpha + Delta K . Radial velocity variations of the white dwarf reveal a semiamplitude of K -WD = 112 +/- 14 km s(-1). The blue spectrum of the white dwarf is well fit ted by a synthetic spectrum having T-eff = 29,300 K and log g = 7.38. The w hite dwarf contributes 97% of the light at 4500 Angstrom and virtually all of the light blueward of 3800 Angstrom. No eclipses are observed. The mass inferred for the white dwarf depends on the assumed mass of the thin residu al hydrogen envelope: 0.40 M. less than or equal to M-WD less than or equal to 0.45 M. for hydrogen envelope masses of 0 M. less than or equal to M-H less than or equal to 4 x 10(-4) M.. We argue that the mass of the white dw arf is closer to 0.45 M.; hence, it appears that the white dwarf has a rela tively large residual hydrogen envelope. The mass of the M star is then M-M = 0.28 +/- 0.05 M., and the inclination is i = 77 degrees +/- 7 degrees. W e discuss briefly how PG 1224 + 309 may be used to constrain theories of cl ose binary star evolution and the past and future histories of PG 1224 + 30 9 itself. The star is both a "post-common envelope" star and a "pre-catacly smic binary" star. Mass transfer by Roche lobe overflow should commence in about 10(10) yr.