Comment on 'e(+)-H- four-particle threshold break-up'

Poelstra et al (1994 J. Phys. B: At. Mel. Opt. Phys. 27 781-94) have presen ted a calculation of the Wannier exponent for the four-particle threshold b reak-up in double ionization of H- by positron impact. These authors find t he threshold power law to he governed by a planar break-up configuration wi th only one unstable mode. This result has recently been questioned by Kuch iev and Ostrovsky (1998 Phys. Rev: A 58 321-35), who find two unstable mode s for the same configuration. The reason for this disagreement remained unc lear. In this comment we show both by an intuitive argument and by a simple calculation that it is an unfortunate reduction of coordinates performed i n the calculation by Poelstra et al that causes this disagreement.