![alt text][1]
Code:
\documentclass[margin=5pt]{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[
kreis/.style={
purple,
line width=1mm,
double=gray!20,
double distance=4mm
},
kurve/.style={kreis,
looseness=1.4
}
]
\newcommand\aR{4cm}
\newcommand\iR{2.5cm}
\newcommand\winkel{10}
% Koordinaten in der Mitte und den Ecken
\path
(0,0) coordinate(M)
foreach \i in {1,2,3}
{(M)+({-30+\i*120}:\aR) coordinate (A\i)};
% Kreis zeichnen
\draw[kreis](M) circle [radius=\iR];
% Bounding Box begrenzen
\draw[kurve,opacity=0]
([yshift=\pgflinewidth]A1)--([xshift=-\pgflinewidth]A2)--([xshift=\pgflinewidth]A3)--cycle;
\clip(current bounding box.north west)rectangle(current bounding box.south east);
% Kurve zeichnen
\draw[kurve] (A3)
foreach \i in {1,2,3} {to [out = {90-\winkel+\i*120}, in = {90+\winkel + \i*120}] (A\i)}
--cycle;
% Selbstüberlappung der Kurve korrigieren
\begin{scope}
\clip (M) rectangle (-\iR,\iR);
\draw[kurve](A3) to [out={210-\winkel}, in={210+\winkel}] (A1);
\end{scope}
\begin{scope}
\clip (M)+(-\iR,0) rectangle +(\iR,-\iR);
\draw[kurve](A1) to [out={330-\winkel}, in={330+\winkel}] (A2);
\end{scope}
\begin{scope}
\clip (M) rectangle (\iR,\iR);
\draw[kurve](A2) to [out={90-\winkel}, in={90+\winkel}] (A3);
\end{scope}
%% Überlappung Kreis und Kurve korrigieren
\begin{scope}
\clip foreach \i in {1,2,3}
{(M)--({-90+\i*120}:\aR)--(A\i)--cycle};
\draw[kreis] (M) circle [radius=\iR];
\end{scope}
\end{tikzpicture}
\end{document}
Die Parameter lassen sich in gewissen Grenzen variieren.
[1]: http://texwelt.de/wissen/upfiles/tw_knoten.png
