PG 0308+096 and PG 1026+002: Two new short period binary stars resulting from common-envelope evolution

We have studied the ultraviolet excess stars PG 0308+096 and PG 1026+002 by means of ultraviolet and optical spectroscopy and spectrophotometry. Both objects have been found to be short-period detached binary systems, consisting of DA white dwarfs and dMe main-sequence companions, and can be understood as the result of prior common-envelope binary evolution. Both systems show strong Ha emission periodically modulated in radial velocity. PG 0308+096 consists of a DA 2 white dwarf (T_eff=26 200±2000 K, log g=7.5±0.4) and a M4.5e main-sequence secondary. The orbital period is P=0.284 309±0.000 021 day with an additional error due to cycle count ambiguity of ±0.002 365 day. Spectroscopic estimates for the masses of the white dwarf and M dwarf are, respectively, script M sign₁=0.39^+0.13_-0.10 script M sign_⊙ and script M sign₂=0.18±0.05 script M sign_⊙. The corresponding inferred range on orbital inclination is i=42°±9°, or i=46°± 10° if the observed radial velocity semiamplitude of the M dwarf, K₂, is corrected for bias. PG 0308+096 shows periodic modulations of Hα emission equivalent width, with a minimum occurring when the M dwarf is at inferior conjunction. If the equivalent width variations are due to reprocessing of the EUV flux from the hot primary in the chromosphere of the secondary, the inclination of the orbital plane to the line of sight is close to 30°; if a portion of the Hα emission is "intrinsic" chromospheric activity on the late-type star, uniform over stellar longitude, the inclination is higher. PG 1026+002 consists of a DA 3 white dwarf (T_eff= 17 600±2000 K, log g=8.1±0.4) and a M4e main-sequence secondary. Its orbital period is P=0.597 257 0±0.000 004 9 day with no cycle count ambiguity. Spectroscopic mass estimates are script M sign₁=0.65^+0.26_-0.21 script M sign_⊙ and script M sign₂=0.22±0.05 script M sign_⊙, corresponding to an inclination bounded at i > 49°, or i > 51° if K₂ is corrected for bias. Equivalent width variations in Hα emission also are seen for PG 1026+002, but because the primary is cooler this cannot be attributed to reprocessing of EUV photons. Stellar activity is thus the favored explanation for the Hα emission, although in this picture there are not be a natural explanation for the phase dependence of the equivalent width variations. These estimates for the stellar and orbital parameters for PG 0308+096 and PG 1026+002 suggest that the orbits will evolve by angular momentum loss through gravitational wave radiation to semidetached configurations in 10¹⁰ and 10¹¹ yr, respectively. These times would each be reduced by a factor of 10 if angular momentum loss due to magnetic braking is significant.