And then they were two: detection of non-thermal radio emission from the
  bow shocks of two runaway stars
  Winds from massive stars have recently been deemed promising sites for
investigating relativistic particle acceleration. Particularly, the resulting
bow shock from the interaction of the winds of runaway stars with interstellar
matter has been observed at multiple wavelengths. Here we investigate the O4If
star, BD+433654, the bow shock of which is, so far, the only one proven to
radiate both thermally and non-thermally at radio frequencies. We also consider
NGC7635 as a bow shock candidate and examine its apex for indications of
thermal and non-thermal radio emission. We observed both sources with the VLA
at 4-8 GHz and 8-12 GHz, and with the Effelsberg telescope at 4-8 GHz. We
analysed data from both telescopes individually and combined, obtained their
spectral index maps and calculated their Spectral Energy Distributions. We
present the first high-resolution maps of radio emission from NGC7635. We find
that both emit non-thermal emission in the radio regime, with the clearest
evidence for NGC7635. Our results are less conclusive for BD+433654, as the
emission from its bow shock becomes weaker and fainter at higher radio
frequencies. Our results extend the previous radio results for the BD+433654
bow shock to higher frequencies. Modelling of our data for both sources shows
that accelerated electrons at the wind termination shock are a plausible source
for the non-thermal radio emission, but energetics arguments suggest that any
non-thermal X-ray and $\gamma$-ray emission could be significantly below
existing upper limits. Enhanced synchrotron emission from compressed Galactic
cosmic rays in the radiative bow shock could also explain the radio emission
from the BD+433654 bow shock but not NGC7635. Non-detection of point-like radio
emission from BD+433654 puts an upper limit on the mass-loss rate of the star
that is lower than values quoted in the literature. [abridged]
