arXiv:2611.14065v1 Announce Type: new Abstract: We derive a posteriori error estimates for parabolic problems discretized by conforming finite elements in space and by the backward differentiation formula of order $k$ in time, with variable step-sizes for $k \le 3$ and constant step-size for $k = 4$. The key ingredient of the analysis is a polynomial $(k+1)$-point time reconstruction of degree $k$ of the approximate solution combined with the elliptic reconstruction. We conclude upper a posteriori error bounds in $L^\infty(L^2) \cap L^2(H^1)$ and in $L^\infty(H^1) \cap H^1(L^2)$. Numerical experiments for $k = 1, \dots, 4$ with linear and quadratic finite elements confirm the optimal orders of the estimators in time and in space.