7 Summary

1. We have presented first deep UBV photometry of the Dwarf Irregular galaxy IC1613 in the Local Group. The selected area is a $5.6\times 5.6$ arcmin field centered on the H II regions in the northeast sector of the galaxy.

2. The blue color-color $(U-B,\ B-V)$ diagram was constructed for the first time making it possible to determine the differential reddening E(B-V)=0.06 with $\sigma=0.02$.

3. The isochrones fit clearly shows the presence of stars from a wide variety of ages. The ages of the blue stars are between 5 and 20 Myrs. Most of the yellow to red stars fainter than M_V = -4.0 are roundly matched by the post main-sequence part of the 60 Myrs to 250 Myrs isochrones. The old age population was reliably detected thus confirming Freedman's (1988a) suggestion for the simultaneous presence of young, intermediate-age and old populations.

4. The theoretical H-R diagram has been constructed. The most massive stars in this area of IC1613 reach $85\ M_\odot$. The young stars closely follow the ZAMS. The red part of the H-R diagram contains some evolved red supegaints between 15 and $40\ M_\odot$ and a large amount of low-mass yellow and red giants. We determined Log$(T_{\rm eff})$ and $M_{\rm bol}$ for all variable stars available in the field and pointed out the peculiar position of two very blue irregulars -V8 and V21.

5. We determined the IMF for the blue stars of equal age. The IMF slope is $\Gamma = -2.0$ with $\sigma = 0.1$. The comparison with the IMF slopes of other LMC, SMC and M31 associations displays no differences within the error range and probably the slope of the IMF is really constant (Massey et al. 1995a,b).

6. Nine of the associations outlined by Hodge (1978) were investigated. The ages of the associations were determined by means of an isochrone fit using the method of Flannery & Johnson (1982) on the assumption that within each association all stars are born coeval. A10 and A14 were found to be the youngest associations with ages of approximately 5 Myr, while A12, A18 and A19 were the oldest ones found, having a common age of approx. 20 Myr. Hodge included the brightest red variable supergiants V32 and V38 in the boundaries of A10 and A14 respectively (Sandage 1971). Taking into account their masses, $M_{\rm bol}$ and Log$(T_{\rm eff})$ as well as the derived age of A10 and A14 we conclude from the results in Massey's paper (1998) that they could be real members of those associations. The comparison with the red supergaints in M 31, M 33 and NGC6822 discussed in the same paper shows that the two supergaints in IC1613 are similar to the red supergaints in NGC6822.

7. The OB stars in our field were re-grouped by cluster analysis. Our present criterion generally divides a Hodge association into several smaller groups thus defining a total of 30 high-density groups. 23 of them cover the Hodge outlines and 7 are completely new ones.

8. Eight Hodge open clusters were identified in our field of observations. Two new cluster candidates were obtained by cluster analysis. The total U, B and V magnitudes were determined for the first time. The observed distribution of clusters in the color-color (U-B, B-V) diagram was compared with the SWB classification of Bica et al. (1996). Eight clusters from our sample fall into the SWB0 type zone (Bica et al. 1992) for very young clusters associated with the H II regions and ages between 0 and 10Myr. Only C12 falls into the SWB1 type zone of clusters around 10Myr. The integrated colors of the clusters belonging to this group may reach very red B-V colors owing to a red supergiant phase.

9. Six new possible nonstellar objects are reported. Most probably they are distant galaxies visible through the disk of IC1613 which confirms Baade's original suggestion that IC1613 contains no bright globular clusters.

Acknowledgements

L.G., J.B. and R.K. would like to thank T. Valtchev and A. Staneva for their help in the process of obtaining the CCD frames and also E. Chelebiev for his help. The authors gratefully acknowledge the useful comments and suggestions raised by Dr. V.D. Ivanov and Dr. N. Spassova, as well as the comments by an anonymous referee. This research was supported by the "Programa de cooperations Mexico-Republica de Bulgaria - "Estrellas Masivas en las Galaxias del Grupo Local" Of. No. 403. S.T.A.C.R.I.25/97. This work was performed while J.B. was a visiting astronomer in UNAM, Mexico under contacts CONACYT No. 400354-5-2398PE and DGAPA INI04696.