1 Introduction

$\delta$ Scuti-type variable stars are pulsating variables located in the lower part of the classical instability strip near or slightly above the main-sequence. The characteristic time-scale of the light variation is in the order of 0 $.\!\!^{\rm d}$1, while the observed light curves are usually multiperiodic due to the simultaneously excited radial and/or non-radial modes. Reliable mode identification requires long-term and/or multi-site observing campaigns, which are well-illustrated, e.g. by recent results from the Delta Scuti Network (Breger et al. 1998; Breger 2000), STEPHI program (Alvarez et al. 1998) or the Whole Earth Telescope (Handler et al. 1997). A handbook of reviews and discussion of the astrophysical importance of these variables has been published very recently (Breger & Montgomery 2000). The most complete catalogue of $\delta$ Scuti stars has been tailored and analysed by Rodríguez et al. (2000) and Rodríguez & Breger (2001).

The light variation of the short period variable V784 Cas (=HD 13122 = BD+59$^\circ$422, P= 0.1092 d, $\Delta V=$ 0 $.\!\!^{\rm m}$06, spectral type F5II, ESA 1997) was discovered by the Hipparcos astrometric satellite. The R00 catalogue (Rodríguez et al. 2000) includes this star listing the parameters derived from the Hipparcos observations. The star lies about 1$^\circ$ NW of the open cluster Stock 2, but it is not associated with this strongly reddened cluster located at 316 pc (Krzeminski & Serkowski 1967). The Hipparcos parallax (9.81 $\pm$ 0.75 mas) supports the close proximity of the star (102 ⁺⁸_-7 pc). A few radial velocity measurements can be found in the literature, they range from -6 km s^-1 (De Medeiros & Mayor 1999) to +20 km s^-1 (Duflot et al. 1995). UBVRI photometry was given by Fernie (1983), while the star was included in the list of bright northern stars with interesting Strömgren indices by Olsen (1980). V784 Cas was also studied in a sample of bright giant stars by Lèbre & De Medeiros (1997), where no emission features, neither time variations or asymmetries of the H$\alpha$ line profiles have been detected (this star was observed two times separated by ten months). The measured rotational velocity is 66 km s^-1 (De Medeiros & Mayor 1999). Neither of the studies mentioned above dealt with the time-dependent phenomena, only the scatter of the velocity measurements (4 km s^-1), as listed in De Medeiros & Mayor (1999), suggested the possible variability. Most recently, Gray et al. (2001) included the star in their large sample of late A-, F- and early G-type stars and determined its spectral type (F0-F2III). They also noted that V784 Cas is a mild Am star, the lines of Sr II $\lambda$4077 and $\lambda$4216 are enhanced. There is no metallicity determination in the literature.

We started a long-term observational project of obtaining follow-up observations of bright, new variable stars discovered by the Hipparcos satellite. We have so far identified a candidate second overtone field RR Lyrae variable (Kiss et al. 1999a), a new high-amplitude $\delta$ Scuti star (Kiss et al. 1999b) and revealed the misclassification of a contact binary (Csák et al. 2000). The main aim of this paper is to present an analysis of new photometric and spectroscopic observations of V784 Cas. The paper is organised as follows: the observations are described in Sect. 2, Sect. 3 deals with the period analysis, while radial velocities are discussed in Sect. 4. Finally, the physical parameters are presented in Sect. 5.