More than 20 years have elapsed since the advent of the 'binary scenario', and many successes have been obtained in its framework: (i) the finding of Ba and C dwarfs, as well as of WIRRing and Abell-35-like systems (relatively wide binaries whose primary has been spun up by mass transfer - possibly wind accretion); (ii) the similarity between orbital elements of (Tc-poor) S stars and barium stars, indicating that these two classes only differ in their effective temperatures, but that their members have followed the same binary evolution; (iii) the finding that yellow symbiotic stars are metal-deficient barium stars.

Nevertheless, several challenges remain: (i) No mass transfer scenario can so far satisfactorily reproduce the observed eccentricity-period diagram, unless dramatic orbital shrinkage during case C of Roche Lab Overflow may be avoided; (i) Why are there no S stars among red symbiotics, and why not all metal-deficient barium stars are symbiotics?

In order to answer the latter question, fast methods to assess the binary nature of stars may be of interest. Some non-conventional, recently-developed methods will be briefly presented: