Light curves
In astronomy, a light curve is a graph of light intensity of a celestial object as a function of time, typically with the magnitude of light received on the y axis and with time on the x axis. Siril is able to generate such curves when analyzing stars.
There are now two ways of selecting the variable and references (also called comparison) stars: manually, or using a list of stars obtained by the N.I.N.A. exoplanet plugin.
Manual star selection
Start by selecting stars and running photometry analysis on the sequence for each, as explained here.

One star is the variable (purple with a V) and the 5 others are used as references.
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Make sure to not select variable stars for references. If the astrometry is done on your image, do not hesitate to use the SIMBAD request to know more about the stars.
Truco
It is preferable to choose references whose magnitude is close to that of the variable.
Once done, Siril automatically loads the Plot tab as shown in the figure below. This shows FWHM curves expressed as a function of frame number.

The plot tab as showed right after the quick photometry on sequence.
What interests us in this part is to display the magnitude curves. Simply go to the drop-down menu and change FHWM to Magnitude. The magnitude curves of each analyzed star are then displayed. This also results in the button Light Curve being sensitive. It is also recommended to check the Julian Date button in order to plot magnitude as a function of a date.

Switching to magnitude view make the Light Curve button sensitive.
Once the analysis is completed with a number of reference stars of at least 4
or 5 (the higher the number, the more accurate the result), you can click on
the Light Curve button. Siril will ask for a file name to save the data
in csv
format, then the light curve will be displayed in a new window. The
csv
file can of course be used in any other software or website to reduce
the data.
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As already mentionned, the software gnuplot must be installed to be able to see light curves.
Light curve of an exoplanet transit.
Commands and automatic operation
It is also possible to automate or create the light curve remotely using the
light_curve
command. As blind operation needs as much automation as
possible, the configuration of the background annulus radii can be automated
with the -autoring
argument: it runs a star detection in the reference
image and multiplies the mean FWHM with a configurabe factor to obtain the
inner and outer radii that should work with the sequence.
Siril command line
light_curve sequencename channel [-autoring] { -at=x,y | -wcs=ra,dec } { -refat=x,y | -refwcs=ra,dec } ...
light_curve sequencename channel [-autoring] -ninastars=file