SIMBAD references

We present photometry in U, B, V, R and I continuum bands and in Hα and Hβ emission lines for a sample of 336 circumnuclear star-forming regions located in early-type spiral galaxies with different levels of activity in their nuclei. They are nearby galaxies, with distances less than 100 Mpc, 60 per cent of which are considered as interacting objects. This survey of 20 nuclear rings aims to provide insight into their star formation properties as age, stellar population and star formation rate. Extinction-corrected Hα luminosities range from 1.3x10³⁸ to 4x10⁴¹erg/s, with most of the regions showing values between 39.5 ≤ logL(Hα) ≤ 40, which implies masses for the ionizing clusters higher than 2x10⁵M_☉. Hα and Hβ images have allowed us to obtain an accurate measure of extinction. We have found an average value of AV = 1.85 mag. (U - B) colour follows a two maximum distribution around (U - B) ≃ -0.7, and -0.3; (R - I) also presents a bimodal behaviour, with maximum values of 0.6 and 0.9. Reddest (U - B) and (R - I) regions appear in non-interacting galaxies. Reddest (R - I) regions lie in strongly barred galaxies. For a significant number of HII regions, the observed colours and equivalent widths are not well reproduced by single burst evolutionary theoretical models.