SY Equ : period changes? Is it BCEP?

orionlee · February 9, 2026, 10:41pm

SY Equ, aka ASAS 212329+0955.9 / Gaia DR3 1745157813523445248 / TIC 288489491, is listed as a BCEP in VSX (based on ASAS-3 Variables) and Gaia DR3 Variables with a period of ~0.16587 d .

It looks like the period has decreased slightly to 0.165799 d in TESS data (in years 2022 / 2024), compared to the period obtained from ASAS-3 (in 2000s) and Gaia DR3 (2010s). See the plot below.
Does the period decrease look real? Or is there some other interpretation (e.g., the TESS data are based on two ~27d high-cadence observations separated by ~2 years, while ASAS-3 / Gaia DR3 data are more evenly spread out)?
Is SY Equ a BCEP (as classified by ASAS-3 Variable / Gaia DR3 Variable) at all? In TESS data, the folded lightcurve looks too “clean” to me (compared to other BCEPs I’ve seen), e.g., there is no modulation of amplitude.

Thanks!

Folded lightcurves of the target from ASAS-3, Gaia DR3, and TESS data.
Left: period = 0.16587 d (best for ASAS-3/Gaia DR3 Variable); right: period = 0.165799 d (best for TESS)

bskiff · February 10, 2026, 3:56pm

…or there is the interplay or two or three (or five or seven!) periods at work?

\Brian

orionlee · February 10, 2026, 5:15pm

Hmm, it looks like a period of ~0.165874 d could fit all data (ASAS-3, Gaia DR3, TESS) reasonably well.

For reference, a 5-day window of JD plot of TESS data.

It looks like there is a single dominant period, along with a small modulation of amplitude.

The single dominant period is also shown in a periodogram of TESS data:

doctor13 · February 10, 2026, 5:28pm

Let me say that watching for Period changes from a Fourier analysis can be a bit trickly. I would normally want to take a look at the O-C values over time to watch for period jumps or constant period changes. The TESS data looks good for getting times of maximum light. Although stitching together a graph with the time gaps can be a challenge. I think the later analysis shows some really good results with a clear f1, 2f1, and 3f1 in the data with a secondary period at just under 7 cycles/day.

Checking other data for SY Equ makes me think that it is a BCep. It has a B-V value of -0.2 and a spectral type of B0.5 III. Both of which are exactly what I would expect from this type of variable. Given these values are the period I would be surprised if it was anything but a BCEP.

Sebastian_Otero · February 18, 2026, 6:19pm

We welcome a VSX revision with this new period!

orionlee · February 20, 2026, 5:05am

When combining all data in one plot, I could not get a single period / epoch that fits all data (Hipparcos, ASAS-3, Gaia DR3 G, TESS) nicely. I wonder if I did not get an accurate enough period, or if there is some genuine period change over time.

The closest I got is 0.1658727 d. ASAS-3 data is misaligned.

Or a slightly smaller 0.1658720 d (ASAS-3 data aligned, but not Hipparcos data):

Note: The 0.165874 d I got in the previous post also results in a misaligned plot.