Measured R-band Afρ for short period comets. Open black diamonds are values found in literature for comets 1P/Halley (Fink 1994), 4P/Faye (Lamy et al. 1996, 2009), 10P/Tempel 2 (Lamy et al. 2009), 17P/Holmes (Lamy et al. 2009), 21P/Giacobini-Zinner (Lara et al. 2003), 32P/Comas Solá (Lowry et al. 1999), 37P/Forbes (Lamy et al. 2009), 44P/Reinmuth (Lamy et al. 2009), 46P/Wirtanen (Fink et al. 1997, Cartwright et al. 1997, Epifani et al. 1999), 50P/Arend (Lowry and Weissman 2003, Lamy et al. 2009), 59P/Kearns-Kwee (Lamy et al. 2009), 63P/Wild 1 (Lamy et al. 2009), 67P/Churyumov-Gerasimenko (Schulz et al. 2004, Lara et al. 2005), 71P/Clark (Lamy et al. 2009), 81P/Wild 2 (Schulz et al. 2003), 84P/Giclas (Lamy et al. 2009), 89P/Russell 2 (Lowry et al. 1999), 106P/Schuster (Lamy et al. 2009), 112P/Urata-Niijima (Lamy et al. 2009), 114P/Wiseman-Skiff (Lamy et al. 2009), 117P/Helin-Roman-Alu 1 (Mazzotta Epifani et al. 2008), 121P/Shoemaker-Holt 2 (Mazzotta Epifani et al. 2008), 137P/Shoemaker-Levy 2 (Lowry et al. 2003). Larger symbols are the values derived in present paper in the innermost 1.4 × 10⁴ km: 32P/Comas Solá at r_h = 2.44 AU (blue filled diamond), 56P/Slaughter-Burnham on 5 October 2004 (green open circle) and on 6 October 2004 (green filled circle), 78P/Gehrels 2 (red filled square). The error bars are of the same order or even smaller than the symbol size.