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Astron. Astrophys. 331, 187-192 (1998)

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1. Introduction

Dwarf Novae (hereafter DNe) are Cataclysmic Variables (CVs) characterized by periodic outbursts, lasting few days, followed by longer periods of quiescence lasting from few weeks to several months. These outbursts (see Cannizzo 1993 and references therein) are originated by cyclic instabilities in the accretion disk surrounding a hot white dwarf (WD) or subdwarf. Z Cam stars form a subclass of DNe in the sense that they are characterized by outburst `standstills' (see Warner 1995), i.e. by prolonged phases in which their luminosity is halfway between maximum and quiescence. According to Osaki (1996), in these systems the mass transfer rate from the secondary is very close to the limit at which the accretion rate from the secondary ceases to trigger periodic instability episodes in the accretion disk and maintains the disk in a stable state (Frank et al. 1992). According to the observations, Z Cam stars are above the Period Gap (Warner 1995).

V1101 Aql is listed in the General Catalog of Variable Stars (Kholopov 1987) as an irregular variable, although Richter (1961) stated that this object was an RR Lyr star. On the contrary, Meinunger (1965), Vogt & Bateson (1982) and Downes & Shara (1993) classified it as a Z Cam-type DN. This would be confirmed by the observations of Pastukhova & Shugarov (1994), which noticed that the star shows an ultraviolet excess, generally varies in the B band between magnitudes 13.8 and 14.8 and has `Algol-like fadings' down to magnitude [FORMULA] (these might possibly be observations made during the quiescent phase). Actually, they classified V1101 Aql as a CV. The same conclusion was reached by Downes et al. (1995) from the analysis of spectra acquired on September 1992 and on August 1994, when the object was at [FORMULA] and [FORMULA], respectively. However, it should be noted that the latter authors find some spectral similarities between V1101 Aql and the class of Herbig Ae/Be stars.

In this paper we present high-time resolution photometry of V1101 Aql obtained on September 1993 and on July 1996, together with a spectrum secured on June 1996. Sect. 2 will describe the employed instruments and the reduction techniques, Sect. 3 will analyse the spectrophotometric data and Sect. 4 will discuss the results. Finally, Sect. 5 will draw our conclusions.

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© European Southern Observatory (ESO) 1998

Online publication: February 4, 1998