PG-0308-1026+002 - 2 NEW SHORT-PERIOD BINARY STARS RESULTING FROM COMMON-ENVELOPE EVOLUTION(096 AND PG)

We have studied the ultraviolet excess stars PG 0308 + 096 and PG 1026 + 002 by means of ultraviolet and optical spectroscopy and spectropho tometry. Both objects have been found to be short-period detached bina ry systems, consisting of DA white dwarfs and dMe main-sequence compan ions, and can be understood as the result of prior common-envelope bin ary evolution. Both systems show strong Halpha emission periodically m odulated in radial velocity. PG 0308 + 096 consists of a DA 2 white dw arf (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 an d M dwarf are, respectively, M1 = 0.39(+0.13/-0.10) M. and M2 = 0.18 /- 0.05 M.. The corresponding inferred range on orbital inclination is i = 42-degrees +/- 9-degrees, or i = 46-degrees +/- 10-degrees if the observed radial velocity semiamplitude of the M dwarf, K2, is correct ed for bias. PG 0308 + 096 shows periodic modulations of Halpha emissi on equivalent width, with a minimum occurring when the M dwarf is at i nferior conjunction. If the equivalent width variations are due to rep rocessing of the EUV flux from the hot primary in the chromosphere of the secondary, the inclination of the orbital plane to the line of sig ht is close to 30-degrees; if a portion of the Halpha emission is ''in trinsic'' chromospheric activity on the late-type star, uniform over s tellar longitude, the inclination is higher. PG 1026 + 002 consists of a DA 3 white dwarf (T(eff) = 17600 +/- 2000 K, log g = 8.1 +/- 0.4) a nd a M4e main-sequence secondary. Its orbital period is P = 0.5972570 +/- 0.0000049 day with no cycle count ambiguity. Spectroscopic mass es timates are M1 = 0.65(+0.26/-0.21) M. and M2 = 0.22 +/- 0.05 M., corre sponding to an inclination bounded at i > 49-degrees, or i > 51-degree s if K2 is corrected for bias. Equivalent width variations in Ha emiss ion also are seen for PG 1026 + 002, but because the primary is cooler this cannot be attributed to reprocessing of EUV photons. Stellar act ivity is thus the favored explanation for the Ha emission, although in this picture there may not be a natural explanation for the phase dep endence of the equivalent width variations. These estimates for the st ellar and orbital parameters for PG 0308+096 and PG 1026+002 suggest t hat the orbits will evolve by angular momentum loss through gravitatio nal wave radiation to semidetached configurations in 1 0(10) and 10(11 ) yr, respectively. These times would each be reduced by a factor of 1 0 if angular momentum loss due to magnetic braking is significant.