13CO(1-0) and 12CO(2-1) in the center of the barred galaxy NGC 1530

We present (CO)-C-13(1-0) and (CO)-C-12(2-1) aperture synthesis maps of the barred spiral galaxy NGC 1530. The angular resolutions are respectively 3 ".1 and 1 ".6. Both transitions show features similar to the (CO)-C-12(1-0) map, with a nuclear feature (a ring or unresolved spiral arms) surrounded by two curved arcs. The average line ratios are (CO)-C-12(1-0)/(CO)-C-13(1-0)= 9.3 and (CO)-C-12(2-1)/(CO)-C-12(1-0)= 0.7. The (CO)-C-12/(CO)-C-13 ratio is lower in the circumnuclear ring (6 - 8) than in the arcs (11 - 15). We fit the observed line ratios by escape probability models, and deduce that the gas density is probably higher in the nuclear feature (greater than or equal to 5x10(2) cm(-3)) than in the arcs (similar or equal to 2x10(2) cm(-3)), confirming earlier HCN results. The kinetic temperatures are in the range 20 - 90 K, but are weakly constrained by the model. The average filling factor of the (CO)-C-12(1-0) emitting gas is low, similar or equal to 0.15. The cm-radio continuum emission also peaks in the nuclear feature, indicating a higher rate of star formation than in the arcs. We derive values for the CO luminosity to molecular gas mass conversion factor between 0.3 and 2.3 M-. (K km s(-1) pc(2))(-1), significantly lower than the standard Galactic value.