J-PLUS photospectrum of the RR Lyrae star of type RRc (α = 123.6246 deg, δ = 31.188 deg). The gray line shows the SDSS spectrum of this star. The J-PLUS photospectrum and the SDSS spectrum were acquired at different pulsation phases ϕ, as labeled in the panel, with ϕ = 0,  1 corresponding to the maximum flux of the source (i.e., the entire period is covered by the ϕ range of values ϕ ∈ [0, 1). The SDSS spectrum was taken not far from its maximum brightness.