  We present photometric CCD observations of stars in four stellar trapezia ADS
15184, ADS 4728, ADS 2843, and ADS 16795. This study is performed on images
obtained at the Observatorio Astron\'omico Nacional at San Pedro M\'artir
(OAN), Baja California, M\'exico. In this work we utilise aperture photometry
to measure the $U$, $B$, $V$, $R$ and $I$ magnitudes of some of the stars in
these dynamically unstable stellar clusters (trapezia).
  Using the $Q=(U-B)-0.72(B-V)$ parameter we obtained the spectral type of the
studied stars as well as their distance to the Sun and their reddening. Slight
differences between the $Q$-derived Spectral types and those listed in SIMBAD
might be due to a different value, from $0.72$, for the slope of the reddening
line on the two-colour diagram.
