Given a planar convex set C, we give sublinear approximation algorithms to determine approximations of the largest axially symmetric convex set S contained in C, and the smallest such set S' that contains C. More precisely, for any epsilon > 0, we find an axially symmetric convex polygon Q subset of C with area vertical bar Q vertical bar > (1 - epsilon)vertical bar S vertical bar and we find an axially symmetric convex polygon Q' containing C with area vertical bar Q'vertical bar < (1 + epsilon)vertical bar S'vertical bar. We assume that C is given in a data structure that allows to answer the following two types of query in time T-C: given a direction u, find an extreme point of C in direction u, and given a line l, find C boolean AND l. For instance, if C is a convex n-gon and its vertices are given in a sorted array, then T-C = O(logn). Then we can find Q and Q' in time O(epsilon T--1/2(C) + epsilon(-3/2)). Using these techniques, we can also find approximations to the perimeter, area, diameter, width, smallest enclosing rectangle and smallest enclosing circle of C in time O(epsilon T--1/2(C)). (c) 2005 Elsevier B.V. All rights reserved.