Scintillation velocities for four millisecond pulsars

A survey for pulsars with the Parkes radio telescope has discovered a number of millisecond pulsars with low dispersion measures. We have measured the scintillation velocities for four of these pulsars, all of which have velocities significantly less than 100 km s-1. Although the velocities of the three binary millisecond pulsars are low compared with the majority of 'normal' pulsars, they remain consistent with their formation through the standard route of supernova explosion followed by a spin-up phase. Selection effects which prevent the detection of millisecond pulsars at distances greater than ∼1 kpc imply that we detect only the lower end of the velocity distribution. PSR J1730-2304, a single millisecond pulsar, has a very low scintillation velocity and may have formed through the merger of two white dwarfs.