  The \textit{Vera C. Rubin} Observatory will start operations in 2025. During
the first two years, too few visits per target per band will be available,
meaning that mean magnitude measurements of variable stars will not be precise
and thus, standard candles like RR Lyrae (RRL) will not be usable. Light curve
templates (LCTs) can be adopted to estimate the mean magnitude of a variable
star with few magnitude measurements, provided that their period (plus
amplitude and reference epoch, depending on how the LCT is applied) is known.
LSST will provide precise RRL periods within the first six months, allowing to
exploit RRLs if LCTs were available.
  We aim to build LCTs in the LSST bands to enhance the early science with
LSST. Using them will provide a 1-2 years advantage with respect to a classical
approach, concerning distance measurements.
  We collected $gri$-band data from the ZTF survey and $z$-band data from DECam
to build the LCTs of RRLs. We also adopted synthetic $griz$-band data in the
LSST system from pulsation models, plus SDSS, \gaia and OGLE photometry,
inspecting the light amplitude ratios in different photometric systems to
provide useful conversions to apply the LCTs.
  We have built LCTs of RRLs in the $griz$ bands of the LSST photometric
system; for the $z$ band, we could build only fundamental-mode RRL LCTs. We
quantitatively demonstrated that LCTs built with ZTF and DECam data can be
adopted on the LSST photometric system. LCTs will decrease by a factor of at
least two the uncertainty on distance estimates of RRLs, with respect to a
simple average of the available measurements. Finally, within our tests, we
have found a brand new behavior of amplitude ratios in the Large Magellanic
Cloud.
