1 Introduction

Havlen-Moffat No. 1 (hereafter HM1) = C1715-387 ( $l = 348.7^{\circ}$, $b = -0.8^{\circ}$) is a compact group of stars that is particularly interesting due to the presence of WR- and Of-type stars (Stephenson & Sanduleak 1971; Sanduleak 1974). Situated in the inner part of our Galaxy, in the spiral II-arm beyond the Sagittarius arm, this cluster is a highly reddened object projected against the HII region RCW 121 (Georgelin & Georgelin 1970; Rodgers et al. 1960). A first attempt to establish the cluster parameters comes from Havlen & Moffat (1977, hereafter HM77) who carried out UBV photometry of over 20 stars and spectroscopy for a handful of them. Lundström & Stenholm (1984) studied the two WR stars whereas Thé et al. (1982; hereafter TAH82) employing Walraven and VRI,JHKL photometry, extended the HM77 work, setting the cluster distance between 2.9 and 3.9 kpc. They suggested that four of the brightest stars have near infrared excesses and that HM1 stars are probably exciting the HII regions RCW 122 and 123. It is curious that this cluster did not receive more attention in the past, despite being a highly reddened object containing stars that are key objects to understand the stellar evolution of massive stars. More recent specific information on the WR stars appears in Crowther et al. (1995), where they were classified as WN7+(abs?), and in Walborn & Fitzpatrick (2000) who re-classified LSS 4065 as a weak line WN8-A star.

Taking into account the importance of the stellar population in HM1, we intend to improve its observational parameters using a large star sample. In the first approach of TAH82, the reddening law in this cluster was assumed as normal; however, this assertion based on VRI,JHKL photometry of only 4 of the bright stars requires further verification with more extensive UBVRI photometry. Besides, to determine a more accurate cluster age and to provide a better distance estimation, a photometric survey including faint stars is clearly needed. On the other hand, with such a survey it is possible to build the luminosity and initial mass functions of the most massive stars in the cluster. These two functions are of crucial astrophysical interest (Miller & Scalo 1978) because they provide us with information on how many of the bright stars have ended their lives as supernovae, how much mechanical energy coming from them was injected in the interstellar medium and how many unobservable faint stars (which are important in the dynamical evolution of this kind of system) are expected to be found (Will et al. 1995).

Figure 1: Finding chart of HM1 showing the positioning of the five frames. The size of the symbols is approximately proportional to V. Some star numberings from HM77 (1 to 24) and TAH82 (starting at 25) are given as a guide. Those in brackets are from the present work. Cluster boundaries are indicated by a 3 ${\hbox {$^\prime $ }}$ radius circle. North and East are also indicated

Bearing this in mind, we decided to perform extensive and deep photometry, including also polarimetric observations of several bright cluster stars. This last type of study was designed to observe the state of polarisation of the interstellar medium towards this highly reddened object. In Sect. 2 we present details of the observations and of the data reduction processes in both photometry and polarimetry; in Sect. 3 we discuss the cluster membership and the reddening law and then we derive the main cluster parameters, distance and age. In Sect. 4 we compute the luminosity and initial mass function of the cluster. Conclusions are presented in Sect. 5.