HD 240121 - an ACV variable showing anti-phase variations of the B and V light curves

TL;DR

HD 240121 is a young, silicon-rich B-type star exhibiting rare anti-phase light variations in B and V bands, confirming its classification as an ACV variable and highlighting the need for further multi-color studies.

Contribution

This study provides a comprehensive analysis of HD 240121, confirming its ACV classification and documenting its unique anti-phase light variations through data mining and archival data analysis.

Findings

HD 240121 shows anti-phase variations in B and V light curves.

The star is a young, silicon CP2 type B star.

Anti-phase variations suggest a null wavelength in the visual spectrum.

Abstract

The variability of HD 240121 = BD+59 2602 was first suspected by S\"arg & Wramdemark (1970) and later confirmed by Gr\"obel (1992a,b). Because of the observed anti-phase variations of the B and V light curves, the latter author tentatively suggested an ACV type. Apart from its inclusion in the catalog of New Suspected Variables (NSV 25977), no further investigations of the star have been published. HD 240121 was included into our target list of ACV candidates and investigated in order to determine the reason for the observed brightness variations. All available information on HD 240121 were collected via an exhaustive data mining procedure. Data from Gr\"obel (1992a,b) were re-analysed and photometric observations from the NSVS and Hipparcos archives were procured and investigated. Line-of-sight reddening and stellar parameters were calculated from archival photometric data. HD 240121 is a young, late B-type CP2 star of the silicon subgroup. The observed period, amplitude of light variations and variability pattern (anti-phase variations) are typical of ACV variables. The occurrence of anti-phase variations of the B and V light curves is rarely observed and points to the existence of a null wavelength in the visual spectrum. We therefore strongly encourage further multi-colour photometric observations of this star.