11 Summary

We have obtained H$_\alpha$ fluxes and luminosities for a sample of 95 galaxies from calibrated observations of 31 HCGs. The sample thus collected comprises 75$\%$ of the accordant galaxies of the observed groups and it represents the largest H$_\alpha$selected sample of HCG galaxies so far having calibrated fluxes. By the estimated $L_{{\rm H}_\alpha}$ we have obtained the star formation rate of the sample galaxies. In a following paper (Severgnini & Saracco 1999) we will combine the results obtained from the data presented here with dynamical, morphological and broad band photometrical data from the literature (Hickson 1982, 1993; Rood & Struble 1994) to show that the H$_\alpha$ luminosity of galaxies and hence their current star formation rate are affected by the dynamics of groups in which they reside.

Further analysis based on these H$_\alpha$ data will allow us to study the rate of interaction and merger phenomena

occurring in HCGs, yielding new insights about the formation and evolution of these systems.

Acknowledgements

We are grateful to E. Recillas and D. Maccagni which have kindly carried out one of the observing runs. We would like also to thank the S. Pedro Martir observatory staff for the helpful support given during observations. PS would like to acknowledge and to thank B. Catinella and S. Molendi for the useful discussions and suggestions given during this work was in progress.