Every few months, a star in the constellation Cancer flares up, a star that is usually invisible even through a moderate telescope. Within a couple of days its brightness increases by tens of times, then slowly returns to normal. This is the dwarf nova YZ Cancri, a binary system in which a white dwarf pulls gas from a normal companion star. During an outburst, the gas disk around the white dwarf heats up and begins to glow, and waves with a period of about two hours appear in its light curve. These waves are called superhumps, and it was previously thought that their shape hardly changes from the beginning to the end of an outburst. New data show that this is not the case.

Imagine a rotating carousel: it spins at one speed, while the horses on it bob up and down at another. Something similar happens in the disk around the white dwarf: the disk itself orbits in 2.08 hours, while its elongated part slowly rotates, adding an extra 4% to the superhump period. As a result, superhumps do not coincide with the orbital period but are slightly longer. In YZ Cancri, this excess is 4.03%, which for dwarf novae near the lower edge of the period gap has been measured for the first time with such precision. Previously, such systems were a blank spot: they are hard to catch during an outburst, and without long observational runs, the superhump shape cannot be discerned.

When the two outbursts recorded by TESS were compared with five ground-based observations over ten years, a clear sequence of three stages emerged. Initially, the superhump has a sawtooth shape: a sharp rise and a smooth decline, like the teeth of a saw. Then the peak splits into two, with one hump becoming higher than the other. By the end of the plateau, the two humps are almost equal in height. This sequence repeats from outburst to outburst, as if the disk goes through the same script each time. Meanwhile, the superhump clock runs irregularly: at the start of the outburst, the times of maxima arrive sometimes earlier, sometimes later than predicted, while in the middle and end of the plateau the deviations become minimal.

The most regular phase occurs during the double-peaked stages. It is then that superhumps can be used as precise clocks to measure the system's parameters. From the refined period of 0.09043 days and the excess of 4.03%, the mass ratio of the components was estimated: the white dwarf is about 5.7 times more massive than the donor star. This value agrees with theoretical models for systems near the lower edge of the period gap, where the orbital period shrinks to a minimum and then begins to grow.

Previously, astronomers did not know whether the superhump shape changes in dwarf novae near the period gap, nor how stable their clocks are. Now it is clear: the profile passes through three recurring stages, and the most reliable times for measurements are the middle and end of the plateau. This sequence may serve as a template for other dwarf novae: if a system shows the same change in shapes, it means its disk goes through the same stages of instability. There is no practical use for everyday life here, but for understanding how matter falls onto white dwarfs, it is an important step. The next time YZ Cancri erupts, astronomers will already know what masks its disk will wear.