  We explore the possibly that either star-spots or pulsations are the cause of
a periodic radial-velocity signal (P~400 days) from the K-giant binary nu
Octantis (P~1050 days, e~0.25), alternatively conjectured to have a retrograde
planet. Our study is based on temperatures derived from 22 line-depth ratios
(LDRs) for nu Oct and twenty calibration stars. Empirical evidence and
stability modelling provide unexpected support for the planet since other
standard explanations (star-spots, pulsations and additional stellar masses)
each have credibility problems. However, the proposed system presents
formidable challenges to planet-formation and stability theories: it has by far
the smallest stellar separation of any claimed planet-harbouring binary
(a_bin~2.6 au) and an equally unbelievable separation ratio (a_pl/a_bin~0.5),
hence the necessity that the circumstellar orbit be retrograde.
  The LDR analysis of 215 nu Oct spectra acquired between 2001--2007, from
which the RV perturbation was first revealed, have no significant periodicity
at any frequency. The LDRs recover the original 21 stellar temperatures with an
average accuracy of 45+/-25 K. The 215 nu Oct temperatures have a standard
deviation of only 4.2 K. Assuming the host primary is not pulsating, the
temperatures converted to magnitude differences strikingly mimic the very
stable photometric Hipparcos observations 15 years previously, implying the
long-term stability of the star and demonstrating a novel use of LDRs as a
photometric gauge. Our results provide substantial new evidence that
conventional star-spots and pulsations are unlikely causes of the RV
perturbation. The controversial system deserves continued attention, including
with higher resolving-power spectra for bisector and LDR analyses.
