We present metallicity dependent transformation equations between UBV and SDSS ugr colours for red giants with synthetic data. The ranges of the colours used for the transformations are 0.400 $\leq(B-V)_{0}\leq$ 1.460, -0.085$\leq(U - B)_{0}\leq$ 1.868, 0.291$\leq(g-r)_{0}\leq$ 1.326, and 1.030$\leq(u - g)_{0}\leq$ 3.316 mag, and cover almost all the observational colours of red giants. We applied the transformation equations to six clusters with different metallicities and compared the resulting $(u - g)_{0}$ colours with the ones estimated by the calibration of the fiducial sequences of the clusters. The mean and standard deviation of the residuals for all clusters are $<Δ(u - g)_0> =$-0.01 and $σ(u - g)_{0} =$0.07 mag, respectively. We showed that interstellar reddening plays an important role on the derived colours. The transformations can be applied to clusters as well as to field stars. They can be used to extend the colour range of the red giants in the clusters which are restricted due to the saturation of SDSS data.
Using the CIFIST grid of CO5BOLD 3D model atmospheres to study the effects of stellar granulation on photometric colours. I. Grids of 3D corrections in the UBVRI, 2MASS, Hipparcos, Gaia, and SDSS systems
Empirical colour--effective temperature relations in the SDSS system from IRFM temperatures of GALAH and APOGEE stars
Yunnan Astronomical Observatories, Chinese Academy of Sciences（中国科学院云南天文台）；Research School of Astronomy and Astrophysics, Australian National University（澳大利亚国立大学天体物理与天文研究学院）；International Centre of Supernovae (ICESUN)（超新星国际中心）；University of the Chinese Academy of Sciences（中国科学院大学）；CAS Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台光学天文重点实验室）
