A photometric comprehensive study of circumnuclear star forming rings:
  the sample
  We present photometry in U, B, V, R and I continuum bands and in H$\alpha$
and H$\beta$ emission lines for a sample of 336 circumnuclear star forming
regions (CNSFR) 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\% 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 H$\alpha$ luminosities range from $1.3\times 10^{38}$ to
$4\times 10^{41} erg s^{-1}$, with most of the regions showing values between
39.5 $\leq log L(H\alpha) \leq$ 40, which implies masses for the ionizing
clusters higher than $2\times 10^{5} M_\odot $.
  H$\alpha$ and H$\beta$ images have allowed us to obtain an accurate measure
of extinction. We have found an average value of A$_V$ = 1.85 magnitudes. (U-B)
colour follows a two maximum distribution around (U-B)$ \simeq$ -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.
