New and existing photometry for the G0 Ia supergiant HD 18391 is analyzed in order to confirm the nature of the variablity previously detected in the star , which lies off the hot edge of the Cepheid instability strip .
Small-amplitude variability at a level of \Delta V = 0.016 \pm 0.002 is indicated , with a period of P = 123 ^ { d } .04 \pm 0 ^ { d } .06 .
A weaker second signal may be present at P = 177 ^ { d } .84 \pm 0 ^ { d } .18 with \Delta V = 0.007 \pm 0.002 , likely corresponding to fundamental mode pulsation if the primary signal represents overtone pulsation ( 123.04 / 177.84 = 0.69 ) .
The star , with a spectroscopic reddening of E _ { B - V } = 1.02 \pm 0.003 , is associated with heavily-reddened B-type stars in its immediate vicinity that appear to be outlying members of an anonymous young cluster centered \sim 10 \arcmin to the west and 1661 \pm 73 pc distant .
The cluster has nuclear and coronal radii of r _ { n } = 3.5 \arcmin and R _ { c } = 14 \arcmin , respectively , while the parameters for HD 18391 derived from membership in the cluster with its outlying B stars are consistent with those implied by its Cepheid-like pulsation , provided that it follows the semi-period-luminosity relation expected of such objects .
Its inferred luminosity as a cluster member is M _ { V } = -7.76 \pm 0.10 , its age ( 9 \pm 1 ) \times 10 ^ { 6 } years , and its evolutionary mass \sim 19 M _ { \sun } .
HD 18391 is not a classical Cepheid , yet it follows the Cepheid period-luminosity relation closely , much like another Cepheid impostor , V810 Cen .