Discovery of parsec-sized dust shells around AFGL 2688 and AFGL 618

We present ISOPHOT linear scans of two carbon-rich proto-planetary nebulae, AFGL 2688 and AFGL 618. The 53' x 3' scans were made with the ISO AOT PHT3 2 using the 120 and 180 mum filters. Both these objects appear to have very bright central point sources together with extremely extended (radius simi lar to 350"-400") dust shells with brightnesses of similar to 10% of the pe ak brightness of the point source. Assuming constant expansion velocities, ages for these dust shells are of the order of 10(5) yr. The infrared inten sities do not fall off smoothly with radius as one would expect for constan t or even steadily increasing mass-loss rates. Rather, AFGL 2688 and AFGL 6 18 both show enhanced emission at 150" and 300" and 160" and 275", respecti vely. These "periodic" enhancements indicate episodic variations in the mas s loss with timescales (few times 10(4) yr) that appear to coincide with th ermal pulses on the asymptotic giant branch. Comparison of these timescales with theoretical calculations suggest a progenitor main-sequence mass of s imilar to3 M-. for both AFGL 2688 and AFGL 618.