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We explore the possibly that either star-spots or pulsations are the cause ofa periodic radial-velocity signal (P~400 days) from the K-giant binary nuOctantis (P~1050 days, e~0.25), alternatively conjectured to have a retrogradeplanet. Our study is based on temperatures derived from 22 line-depth ratios(LDRs) for nu Oct and twenty calibration stars. Empirical evidence andstability modelling provide unexpected support for the planet since otherstandard explanations (star-spots, pulsations and additional stellar masses)each have credibility problems. However, the proposed system presentsformidable challenges to planet-formation and stability theories: it has by farthe 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, fromwhich the RV perturbation was first revealed, have no significant periodicityat any frequency. The LDRs recover the original 21 stellar temperatures with anaverage accuracy of 45+/-25 K. The 215 nu Oct temperatures have a standarddeviation of only 4.2 K. Assuming the host primary is not pulsating, thetemperatures converted to magnitude differences strikingly mimic the verystable photometric Hipparcos observations 15 years previously, implying thelong-term stability of the star and demonstrating a novel use of LDRs as aphotometric gauge. Our results provide substantial new evidence thatconventional star-spots and pulsations are unlikely causes of the RVperturbation. The controversial system deserves continued attention, includingwith higher resolving-power spectra for bisector and LDR analyses.

Line-depth-ratio temperatures for the close binary nu Octantis: new evidence supporting the conjectured retrograde planet