  We have used H alpha narrow-band imaging to search for star-forming satellite
galaxies around 143 luminous spiral galaxies, with the goal of quantifying the
frequency of occurrence of satellites resembling the Magellanic Clouds, around
galaxies comparable to the Milky Way. For two-thirds of the central galaxies,
no star-forming satellites are found, down to luminosities and star-formation
rates well below those of the Magellanic Clouds. A total of 62 satellites is
found, associated with 47 of the central galaxies searched. The R-band
magnitude difference between central galaxies and their satellites has a median
value of 4.6 mag, and a maximum of 10.2 mag. The mean projected separation of
the satellites from their central galaxies is 81 kpc, or 98 kpc for systems
beyond 30 Mpc. Thus star-forming satellites are quite rare, and the Milky Way
is unusual both for the luminosity and the proximity of its two brightest
satellites. We also find that the Clouds themselves are unusual in that they
appear to form a bound binary pair; such close satellite pairs, of any
luminosity, are also extremely rare in our survey.
