1 Introduction

$\delta$ Scuti and $\gamma$ Doradus type pulsating stars are very important objects for the investigation of the internal structure and evolution theory of intermediate-mass stars from the asteroseismological approach (see Breger 2000 and Zerbi 2000 for recent reviews). $\delta$ Scuti stars are main sequence stars of spectral types from A3 to F0, having short pulsational periods of 0.02 to 0.3 day and solar metal abundance. They lie on or above the main sequence facing on an extension of Cepheid instability strip in the Hertzsprung-Russell diagram. Their pulsational characteristics show slight differences according to their evolution stages (Breger 1991). Stars in the hydrogen shell burning stage have generally one or two periods with pure radial modes and high amplitudes of more than 0 $.\!\!^{\rm m}$1. In comparison with these high amplitude $\delta$ Scuti stars (so called, HADS), stars in the hydrogen core burning stage near the zero-age main sequence have multi-frequencies with radial modes as well as non-radial ones and small amplitudes of several milli-magnitude.

$\gamma$ Doradus stars are early F-type main sequence or sub-giant stars which have longer periods of 0.4$\sim$3.0 day than that of $\delta$ Scuti stars and amplitudes of less than 0 $.\!\!^{\rm m}$1 in V-band. Although the variability of $\gamma$ Doradus itself was discovered about 40 years ago by Cousins & Warren (1963), $\gamma$ Doradus type stars have been only recently classified as a new type of pulsating variable. The reason is that only a few stars with similar characteristics have been known and their variability was not understood theoretically until a few years ago. They are located around the cool edge of $\delta$ Scuti instability strip (Kaye et al. 1999). Based on the Hipparcos photometry data, Handler (1999) defined the $\gamma$ Doradus instability strip as ranges of 7200$\sim$7700 K on the zero-age main sequence and of 6900$\sim$7500 K with one magnitude brighter than it. They are thought to be excited in high-order (n) low-degree (l) non-radial gravity-modes.

A number of pulsating stars such as $\delta$ Scuti, $\gamma$ Doradus, $\beta$ Cephei and slowly pulsating B-type stars were found in young- or intermediate-age open clusters. Frandsen & Arentoft (1998) discovered seven $\delta$ Scuti stars in NGC 1817 and Freyhammer et al. (2001) found 13 $\delta$ Scuti stars in the central field of NGC 7062. Especially, Arentoft et al. (2001) detected 17 new variable stars in NGC 6231, including three $\delta$ Scuti stars, three $\gamma$ Doradus star candidates, three slowly pulsating B-type star candidates and two $\beta$ Cephei stars. One very good recent review article for this topic is the paper by Rodríguez & Breger (2001). Using $\delta$ Scuti stars in open clusters, Suarez et al. (2002) investigated correlation between the oscillation amplitudes and stellar parameters of $\delta$ Scuti stars.

This study is part of the photometric survey of variable stars in open clusters, which has been carried out at the Bohyunsan Optical Astronomy Observatory (BOAO) in Korea. General descriptions and the most recent results for the survey are given in Kim et al. (2001a, b). We selected an intermediate-age open cluster NGC 2539 (RA $_{2000} = 8^{\rm h} 10^{\rm m} 42^{\rm s}$, Dec ₂₀₀₀ = -12$^\circ$50$^\prime$) at the constellation Puppis in the southern sky. This cluster is classified as II 1 m type (Ruprecht 1966), in which stars are moderately concentrated. The cluster is appropriate for our less-wide CCD camera. It has a lot of A  $\sim$  F type main sequence stars which are thought to be $\delta$ Scuti and $\gamma$ Doradus candidates. The observations and data reduction are described in Sect. 2. We present physical parameters of this cluster and properties of newly detected variable stars in Sect. 3.