The post-common envelope and pre-cataclysmic binary pg 1224+309¹

We have made extensive spectroscopic and photometric observations of PG 1224 + 309, a close binary containing a DA white dwarf primary and an M4+ secondary. The Hα line is in emission due to irradiation of the M star by the hot white dwarf and is seen to vary around the orbit. From the radial velocities of the Hα line we derive a period of P = 0.258689 ± 0.000004 days and a semiamplitude of K_Hα = 160 ± 8 km s^-1. We estimate a correction ΔK = 21 ± 2 km s^-1, where K_M = K_Hα + ΔK. 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 fitted by a synthetic spectrum having T_eff = 29,300 K and log g = 7.38. The white dwarf contributes 97% of the light at 4500 Å and virtually all of the light blueward of 3800 Å. No eclipses are observed. The mass inferred for the white dwarf depends on the assumed mass of the thin residual hydrogen envelope: 0.40 M_⊙ ≤ M_WD ≤ 0.45 M_⊙ for hydrogen envelope masses of 0 M_⊙ ≤ M_H ≤ 4 × 10^-4 M_⊙. We argue that the mass of the white dwarf is closer to 0.45 M_⊙; hence, it appears that the white dwarf has a relatively 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° ± 7°. We discuss briefly how PG 1224 + 309 may be used to constrain theories of close binary star evolution and the past and future histories of PG 1224 + 309 itself. The star is both a "post-common envelope" star and a "pre-cataclysmic binary" star. Mass transfer by Roche lobe overflow should commence in about 10¹⁰ yr.