2000MNRAS.312...70B

We present spectra in the range 8000-8665Å of the close, detached M-dwarf-white-dwarf binaries EG UMa and PG 1026+002. The Nai absorption doublet (8183, 8195Å) is detected along with a number of other absorption lines including intrinsically narrow metal Fei and Tii lines and the Caii infrared (IR) triplet (8498, 8542, 8662Å). We update the ephemeris and orbital period of EG UMa. The radial velocity semi-amplitude for the secondary star in EG UMa was found to be 125.9±0.4km.s^–1. The excellent agreement between this value and that measured from emission lines, in conjunction with the lack of orbital-phase variation of the absorption line strengths, confirms that the secondary-star emission in EG UMa is chromospheric in nature and not influenced by irradiation. We determine a range of rotational velocities, V_rotsini, for EG UMa as measured over the Nai absorption doublet and the Fei and Tii metal lines using three template stars. Although a broad range of V_rotsini values are obtained, the narrow metal lines were found to give consistantly lower values than that given by the Nai absorption doublet. In addition, the measured values of V_rotsini were inconsistent with other measured parameters and were therefore deemed too unreliable to be the basis for full system parameter calculations. We do, however, derive a complete set of system parameters for EG UMa based upon our new measurement of K₂ and previous observational data.

We present R-band relative photometry of EG UMa, which shows relative flux variations at around the 5 per cent level and the occurrence of a flare event. These observations are again interpreted as being the result of the intrinsic chromospheric activity of the M dwarf in this system.

We measure the semi-amplitude of the radial-velocity curve for PG 1026+002 to be 163.7±7.3km.s^–1, which, being in agreement with that measured from emission lines, adds further support to the suggestion that the secondary-star emission in PG 1026+002 is also largely chromospheric in nature. Rotational velocities for PG 1026+002 were determined by measurement over the Nai absorption doublet only. The V_rotsini values obtained for PG 1026+002 using the three available template stars were all greater than expected from the assumption of synchronous rotation.