1 Introduction

The very young open cluster NGC 6231 ($l=345.5^{\circ}$, $b=1.2^{\circ}$) is the nucleus of the association Sco OB1 and is responsible of exciting the nebula RCW 113 (Laval 1972) located in the southern part of the association. Its stellar content includes WR-, $\beta$ Cepheid- and several Of- and O-type stars, many of which show variability and/or have a high chance of being binary systems (Levato & Morrell 1983; Raboud 1996).

This cluster and the overall region of Sco OB1 have been the subject of a comprehensive analysis by Perry et al. (1990, 1991, 1992) from a large compilation of photometric and spectroscopic data. Not much information regarding the cluster lower main sequence structure and the presence of PMS stars can be drawn from this compilation because of its low magnitude limit. Notwithstanding, the authors conclude that all the stars with $V\geq 10$ mag are in a different evolutionary status (pre main sequence stars, PMS) respect of the brightest members. It is timely to recall that in an early investigation of PMS stars, Eggen (1976) suggested that a violent star formation process had taken place there.

Stellar formation seems to be an ongoing process in Sco OB1. Let us mention that magnitude spread at constant colour (currently associated to the presence of PMS stars) was reported in Tr 24, another cluster of this association, by Heske & Wendker (1984, 1985) and that a subgroup of probable PMS stars lying in the field of Sco OB1 was also investigated by Heske & Wendker (1985) and studied in more detail by Piers et al. (1992).

The aim of the present work is to describe the lower cluster structure, looking for PMS evidences, and also to analyse the luminosity and mass distributions of the more massive members. Although broad band photometry does not provide conclusive evidence, on a well established physical ground on the PMS problem, it allows us to inspect the distribution of very faint stars and to look for hints of their presence (e.g. large magnitude spreads at constant colour and star excesses). Firm conclusions on PMS stars need to be supported by, at least, infrared observations and kinematics studies. Two extensive photometric surveys in the area of NGC 6231 have been reported: one by Seggewiss (1968) and another by Raboud et al. (1997). Unfortunately, they are not deep enough to allow a description of the lower main sequence of this cluster, but a new article by Sung et al. (1998) containing $UBVRIH\alpha$ photometry and reporting the detection of PMS stars and PMS candidates here was published. This is an exciting result as, until now, few evidence of stars in contraction towards the ZAMS had been found in open clusters (e.g. Hillenbrand et al. 1993 in NGC 6611). Since there is a comparable photometric limit between Sung et al.'s (1998) study and ours we will analyse some of their findings in relation to ours.