Orbital period of the IW And-type star ST Cha

TL;DR

This study determines the orbital period of the IW And-type dwarf nova ST Cha using TESS data, revealing variability in orbital signal strength and suggesting possible disk state changes related to outbursts.

Contribution

First measurement of ST Cha's orbital period with TESS, showing variability in orbital signal strength and proposing a link to disk state transitions.

Findings

Orbital period of 0.285360 days identified.

Orbital signal strength varies independently of mean brightness.

Outburst with a shoulder may trigger state transition.

Abstract

We analyzed TESS data of the IW And-type dwarf nova ST Cha during ordinary dwarf nova states. We have identified an orbital period of 0.285360(1) d. The object was reported to be eclipsing in the past using the data obtained in the 1970s, which is not in agreement with the present data. Despite the constant mean brightness, the strength of the orbital signal varied significantly, suggesting that the strength of the orbital signal does not always reflect the mass-transfer rate. During an outburst with a shoulder, we did not find evidence of humps recurring with a period longer than the orbital period which were recorded in V363 Lyr. This finding strengthens the idea that V363 Lyr is an unusual object. We found that the strength of the orbital signal increased after an outburst with a shoulder. This outburst may have changed the state of the disk and the hot spot became more apparent. Such a change in the disk may have triggered a transition from an ordinary dwarf nova-type state to an IW And-type state and this possibility would require further examination.