1 Introduction

The present paper is part of an extensive effort to study short-period variable stars in the young open cluster NGC6231. This includes searching for new and confirming known variable stars and monitoring the long-term stability of the variations - i.e. investigate amplitude and frequency variations in a rich set of variable stars of different types. The synopsis of this project is given by Sterken et al. (2002).

NGC6231 has been intensively studied in the past decades, as recently reviewed by Sung et al. (1998, hereinafter SBL). They carried out UBVRI and H$\alpha$ photometry for a number of stars in the cluster in order to determine cluster parameters and search for pre-main sequence (PMS) stars. Baume et al. (1999, hereinafter BVF) carried out a similar photometric UBVI study and the results were found to be in excellent agreement with SBL. BVF found an age of the cluster of 3-5 Myr.

Six $\beta$ Cephei stars are known in the cluster (Balona & Shobbrook 1983, BS83; Balona & Engelbrecht 1985, BE85). Balona & Laney (1995, BL95) searched for additional short-period variables and found one foreground and one possible cluster member $\delta$ Scuti star and 3 eclipsing variables. No additional pulsating B-stars were found ($\beta$ Cephei or Slowly Pulsating B (SPB) stars). Furthermore, a number of spectroscopic-binary stars have been found in the cluster (García & Mermilliod 2001; Raboud 1996; Levato & Morrell 1983).

Finding new pulsating stars of different types in NGC6231 (or any open cluster) is important, as it allows for comparative studies of different classes of pulsating stars originating from common initial conditions.

In this paper we present the results of a search for new variable stars in NGC6231 using CCD time-series data. A part of the observations was optimised for the comparatively bright ($\sim$9$^{\rm m}$) $\beta$ Cephei stars and another part for stars of 13$^{\rm m}$-14$^{\rm m}$. The latter part allows us to search for possible cluster $\delta$ Scuti and $\gamma$ Doradus stars, while in those "deep'' data, the $\beta$ Cephei stars are, naturally, overexposed. We discuss the newly detected variable stars individually and give tentative frequency solutions as indicators of the time scales and amplitudes of the variations.

 

 

Table 1: Data sets discussed in this paper. D90 is the Dutch 0.90-m telescope, D154 the Danish 1.54-m telescope, both at ESO, La Silla, Chile. Comment deep indicates that the observations were optimised for the fainter stars of the cluster, as discussed in the text.

Run	HJD	Observer	Telescope	#nights	#useful images	filters	comment
1998 March	2450887-891	CS	D90	4	62	V	deep
1998 April	2450918-922	TA	D90	5	277	V	deep
1998 June	2450970-985	LF, JC	D154	10	921	V	deep
1999 March	2451263-269	CS	D90	4	150	V	deep
2000 June	2551721-722	HD	D154	2	240	uvby
2000 July	2451737-744	EP, CD	D154	7	1050	uvby