OU Gem (HD 45088)

OU Gem is a bright (V= 6.79, Strassmeier et al. 1990) and nearby (d= 14.7 pc, ESA [1997]) BY Dra-type SB2 system (K3V/ K5V) with an orbital period of 6.99 days and a noticeable eccentricity (Griffin & Emerson []). Both components show Ca  II H & K emission, though the primary shows slightly stronger emission than the secondary. The H$\alpha$ line is in absorption for the primary and filled-in for the secondary (Bopp [1980]; Bopp et al. [], b; Strassmeier et al. 1990; Montes et al. 1995a, b, 1996). Dempsey et al. ([1993a]) observed that the Ca  II IRT lines were filled-in. This binary was detected by the WFC on board the ROSAT satellite during the all-sky survey (Pounds et al. 1993; Pye et al. 1995). OU Gem has 1.7$\times$10²⁹ ergs^-1X-ray luminosity, typical value of the BY Dra systems (Dempsey et al. [1993b], [1997]). The photometric variability was discovered by Bopp et al. ([]) and they also computed a 7.36-day photometric period. It is interesting that the orbital and rotational periods differ in 5% due to the appreciable orbital eccentricity (e= 0.15), according to Bopp ([1980]). Although BY Dra systems are main-sequence stars, their evolutionary stage is not clear. OU Gem has been listed by Soderblom et al. (1990) and Montes et al. ([2000a], [2000c]) as a possible member of the UMa moving group (300 Myr), indicating that it may be a young star.

The H$\alpha$ line: In the observed spectra, we see an absorption line for the primary star and a nearly complete filling-in for the secondary star. After applying the spectral subtraction technique, clear excess H$\alpha$ emission is obtained for the two components, being stronger for the hot one (see Fig.  upper panel). The excess H$\alpha$ emission EW is measured in the subtracted spectrum and corrected for the contribution of the components to the total continuum. We took one spectrum in this region in Dec-92 (Montes et al. 1995b). At the orbital phase of this observation ($\varphi$= 0.48) we could not separate the emission from both components and we measured the total excess H$\alpha$ emission EW relative to the combined continuum. We obtained a similar value to Mar-96, Apr-98 and Jan-99 values obtained adding up the excess emission EW from the two components. The H$\beta$ line: Looking at the observed spectra, we only see the H$\beta$ line for the primary, in absorption. After applying the spectral subtraction technique small excess H$\beta$ emission is obtained for the two components (see Fig.  lower panel). We have obtained, in general, $\frac{ E_{H\alpha} } { E_{H\beta} }$ values larger than three for the two components, so the emission can come from prominences.

The Ca  II H $\&$ K and H$\epsilon$ lines: We observe that both components of this binary have the Ca  II H & K and H$\epsilon$ lines in emission. We can also see that the excess Ca  II H & K emission of the hot star is larger than the one of the cool star (Fig.  upper panel). The measured excess Ca  II H & K emission of both components is larger in the two spectra of the NOT98 observing run than in the NOT96 spectrum. Overlapping between the H$\epsilon$ line of one star and the Ca  II H line of the other only allows to see the H$\epsilon$ line of the cool star at orbital phase 0.19, and H$\epsilon$ of the hot star otherwise.

The Ca  II IRT lines: In the observed spectra, we can see that both components of OU Gem show the Ca  II IRT lines in emission superimposed to the corresponding absorption. After applying the spectral subtraction technique, clear excess emission appears for the two components, being clearly stronger for the hot one (see Fig.  lower panel).