The key component of Buchla style Low Pass Gates and many other exotic Buchla/Serge derived designs.
The C3 is the ‘standard’ Vactrol to use in a Buchla LPG, the C4 is a really nice contrast with a longer tail and often more saturated sound due to the longer sustain.
Excelitas Vactrol analogue optoisolators have an LED as the input element and a photoconductive cell as the output element, the coupled pair acts as an electrically variable potentiometer. These are the parts previously known under the brand name VACTEC, as manufactured by Perkin Elmer.