1 Introduction

NGC 6604 ( $\alpha_{2000} =18^{{\rm h}}~18\hbox{$.\!\!^{\rm m}$ }1$, $\delta_{2000}=-12^{\circ}~14^{\prime}$, $l~=~18\hbox{$.\!\!^\circ$ }3$, $b~=~+1\hbox{$.\!\!^\circ$ }7$) is a fairly compact open cluster belonging to class I3p (Ruprecht 1966) and lies at the core of the HII region S54 (Georgelin et al. 1973). The fairly large extinction to the cluster ( $A_{V} \sim 3$) is surely linked to the partnership with an HII region. The cluster contains several OB type stars (Stephenson & Sanduleak 1971), which suggest the cluster to be fairly young (Forbes & Dupuy 1978). Its distance is rather uncertain, with estimates ranging from 0.7 to 4.4 kpc (Alter et al. 1970). Moffat & Vogt (1975, hereafter MV) presented photoelectric photometry for a dozen stars in the cluster field, deriving a 1.6 kpc distance and a E_B-V=1.01 reddening. Forbes and DuPuy (1978, hereafter FD) performed photoelectric and photographic photometry, which yielded a 2.1 kpc distance, a E_B-V = 0.96 reddening and a 4 10⁶ years age. They pointed out the possible presence of pre-ZAMS members which demanded confirmation.

In this paper we report about our CCD $BVI_{\rm C}$ photometry of the cluster central area, medium resolution spectroscopy of a field larger than the cluster and high resolution Echelle spectroscopy of the brightest cluster members.