The observation of gravitational waves from the compact binary coalescences (CBC) opens a new era of gravitational-wave physics and multi-messenger astronomy. What happens after the coalescence? What else can we learn from the remnants? In this talk, I will introduce two studies about the CBC remnants. One is to search for gravitational waves from the remnant of the binary neutron star merger, GW170817. Although no significant signal candidate is found for this particular event, development and verification of new methods is of importance for future applications. Prospects will become good with future detectors and the combination of multiple channels. The other is to search for yet-undiscovered, weakly interacting, ultralight bosons by following up the newly born black holes after the coalescences. The number of such particles around a rapidly rotating black hole can grow exponentially due to the phenomenon of superradiance, forming a macroscopic cloud and generating continuous gravitational waves. The theory of boson clouds around black holes and the gravitational-wave emission mechanism will be explained. The search challenges and the empirical estimates of the search sensitivity will be discussed. These gravitational-wave searches are closely connected to theoretical predictions and electromagnetic observations. One of the most exciting aspects of gravitational-wave discovery is that pieces from different channels are being brought together to study fundamental physics.