1 Introduction

The Universidad Complutense de Madrid Survey (UCM Survey List I; Zamorano et al. [1994], List II; Zamorano et al. [1996]) constitutes a representative and fairly complete sample of current star-forming galaxies in the Local Universe (Gallego [1999]). Its main purposes are to identify and study new young, low metallicity galaxies and to quantify the properties of the current star formation in the Local Universe. Another key goal was also to provide a reference sample for the studies of high-redshift populations, mainly dominated by star-forming galaxies.

The UCM Survey was carried out with the 80/120 cm f/3 Schmidt telescope at the German-Spanish Observatory of Calar Alto (Almería, Spain). A $4\hbox{$^\circ$ }$ full-aperture prism and a IIIaF photographic emulsion were the standard instrumental setup. The survey was able to detect emission line galaxies (ELG) to a Gunn r magnitude limit of about 18$^{\rm m}$; the objects were selected by the presence of H$\alpha$ $\lambda$6563 + [NII] $\lambda$6584 emission in their spectra. A total number of 191 objects were catalogued as UCM galaxies in List I and List II. A third list (UCM Survey List III; Alonso et al. [1999]) will extend the sample around to 0.5$^{\rm m}$ fainter objects due to the implementation of a new fully automatic procedure for the detection and analysis of the objective-prism spectra.

The galaxies included in UCM lists I and II (hereafter the UCM survey) have been deeply analyzed in the Gunn r bandpass (Vitores et al. [1996a] and [1996b]), and also in the J and K nIR bands by Alonso-Herrero et al. ([1996]) and Gil de Paz et al. ([1999]). The spectroscopic analysis was performed by (Gallego et al. [1996], [1997]). It has also been used to deduce the H$\alpha$ luminosity function in the Local Universe (Gallego et al. [1995]). The UCM sample is now widely used as reference for spectroscopic studies of high-z populations (see the nice review by Madau [1999]).

The UCM survey includes a total of 191 galaxies at an averaged redshift of 0.027. Morphologically, the sample is dominated by late-type spirals (around 47% being Sb or later) with less than 10% presenting typical parameters of earlier types and the remaining 10% being irregulars (Vitores et al. [1996a]). Spectroscopically, all types of star-forming galaxies previously known in the literature are represented; most of the UCM objects are low-excitation, high-metallicity starburst-like galaxies (57%) but there are also high-excitation, low-metallicity HII-like galaxies (32%). A fraction of AGN objects are also present (8%). Their metallicities range from solar values to $\frac{1}{40}Z_{\hbox{$\odot$ }}$, peaking at $\frac{1}{4}Z_{\hbox{$\odot$ }}$ (Gallego et al. [1997]). Photometrically, the UCM Survey was first imaged in the Gunn r band due to the close relationship between this band and the one used in the primary photographic plates. In order to get colour information of this sample, we started a long-term project to obtain detailed B band photometry. This band was selected with two main purposes: (1) obtaining an optical colour with a considerable base width, (2) getting information more directly comparable with high-redshift surveys.

In this paper we present B band photometry for the whole sample and compare it with the previous optical data. In later papers we will perform the study of the disk and bulge components in the B-band and will combine the broad band data (both optical and nIR) with H$\alpha$ images in order to carry out a spatially resolved stellar population synthesis.

The paper is structured as follows: we introduce the sample of galaxies and the Johnson B observations in Sect. 2. The galaxy photometry is afforded in Sect. 3. Statistics and the comparison with previous photometry are considered in Sect. 4. Finally, we present the conclusions in Sect. 5. A Hubble constant H₀=50kms^-1Mpc^-1 and a deceleration parameter q₀=0.5 have been used throughout this paper.