Henrietta Leavitt's classical Cepheid relation

The Leavitt & Pickering (1912) mean period-magnitude relation for SMC Cepheids exhibits correlated residuals relative to modern Gaia B G and Johnson V photometry (r δV/δL12 about0.76). Therefore dispersion inherent to Leavitt's photographic photometry relation is not solely random scatter (σ L12,B G,V=0.22 plus or minus0.03,0.25 plus or minus0.04,0.20 plus or minus0 m.03), and is potentially consistent with the partial encoding of combined effects from the elongated SMC, classical Cepheid instability strip, differential reddening, etc. OGLE data indicate that 870 SMC variables identified by Leavitt (1908, Table 2) are classical Cepheids (90 %), and the preliminary unphased Table 2 photometry likewise features correlated residuals relative to contemporary observations (r δB G/δL08-T2 about0.54). Table 6 data in Leavitt (1908, n=16) define a highly linear Leavitt law over an extensive baseline (r about-0.98), but constitute a smaller sample than the later 1912 work (n=25).