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Astron. Astrophys. 322, 155-158 (1997)

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1. Introduction

Asteroseismology of pulsating white dwarfs offers the opportunity to infer their internal structure and to constrain our current ideas on the still controversial final stages of stellar evolution. However, our knowledge of these late pulsating phases relies on a small sample of known variables. The coolest instability strip, defined by the ZZ Ceti variables or DAV, contains only 24 stars. The two other instability strips for the DBV and PG1159, located at higher effective temperature in the HR diagram, are even less populated. In addition to the work devoted to a better understanding of the physics involved in these non radial pulsators, a continuous search for new variable white dwarfs of the various types is needed. Surveys, either from the ground or from space, have proven to be interesting sources of white dwarf and subdwarf stars, even when their main goal was to search for quasars. One of the most famous examples is the Palomar-Green survey (PG) which almost doubled the number of known white dwarfs and allowed the discovery of some new variable ones (Green, Schmidt, and Liebert 1986). Among the most recent discoveries of new pulsating white dwarfs, the Edinburgh-Cape survey provided two new ZZ Ceti variables: EC23487-2424 (Stobie et al. 1993) and EC14012-1446 (Stobie et al. 1995), increasing appreciably the fraction of the known ZZ Ceti stars in the South hemisphere. The Rosat sky survey has revealed the brightest pulsating PG1159 type star RXJ2117+3412 (Vauclair et al. 1993).

Pursuing our search for new pulsating white dwarfs, we have selected a few candidates from the Kiso survey (Noguchi, Maehara and Kondo 1980; Kondo, Noguchi and Maehara 1984). This survey, whose goal was to detect ultraviolet-excess objects, contains a number of white dwarfs previously not known. In a series of papers, Wegner and collaborators have obtained spectroscopy of about 650 objects from the Kiso survey, allowing their classification and the identification of new white dwarfs (Wegner and McMahan 1985, 1986, 1988; Wegner, McMahan and Boley 1987; Wegner and Swanson 1990a, b; Wegner and Boley 1993). For a sub-sample of spectroscopically identified objects, Wegner et al. (1990) obtained photoelectric photometry from which (B-V) and (U-B) color indices can be derived. We have selected among the DA white dwarfs in this list those with 0.12 [FORMULA] (B-V) [FORMULA] 0.27 and -0.72 [FORMULA] (U-B) [FORMULA] -0.48. These color indices intervals are determined from colors of the known ZZ Cetis, accounting for a generous uncertainty in the photometric measurements. Stobie et al. (1995) have restricted the range of their search to the more severe interval 0.15 [FORMULA] (B-V) [FORMULA] 0.25.

We report the discovery of two new variables from this selected sample: KUV08368+4026 and KUV11370+4222.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998